diff --git a/Cargo.toml b/Cargo.toml
index 5185872e82..6fbb591721 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -24,25 +24,26 @@ appveyor = { repository = "chronotope/chrono" }
 name = "chrono"
 
 [features]
-default = ["clock"]
-clock = ["time"]
+default = ["clock", "std"]
+alloc = []
+std = []
+clock = ["time", "std"]
 wasmbind = ["wasm-bindgen", "js-sys"]
 
 [dependencies]
-libc = { version = "0.2", default-features = false }
 time = { version = "0.1.39", optional = true }
 num-integer = { version = "0.1.36", default-features = false }
 num-traits = { version = "0.2", default-features = false }
 rustc-serialize = { version = "0.3.20", optional = true }
-serde = { version = "1", optional = true }
+serde = { version = "1.0.99", default-features = false, optional = true }
 
 [target.'cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))'.dependencies]
 wasm-bindgen = { version = "0.2", optional = true }
 js-sys = { version = "0.3", optional = true } # contains FFI bindings for the JS Date API
 
 [dev-dependencies]
-serde_json = { version = "1" }
-serde_derive = { version = "1" }
+serde_json = { version = "1", default-features = false }
+serde_derive = { version = "1", default-features = false }
 bincode = { version = "0.8.0" }
 num-iter = { version = "0.1.35", default-features = false }
 doc-comment = "0.3"
diff --git a/Makefile b/Makefile
index bb938ccea5..46b39dd25f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,8 @@
 # this Makefile is mostly for the packaging convenience.
 # casual users should use `cargo` to retrieve the appropriate version of Chrono.
 
+CHANNEL=stable
+
 .PHONY: all
 all:
 	@echo 'Try `cargo build` instead.'
@@ -20,11 +22,8 @@ README.md: src/lib.rs
 
 .PHONY: test
 test:
-	TZ=UTC0 cargo test --features 'serde rustc-serialize bincode' --lib
-	TZ=ACST-9:30 cargo test --features 'serde rustc-serialize bincode' --lib
-	TZ=EST4 cargo test --features 'serde rustc-serialize bincode'
+	CHANNEL=$(CHANNEL) ./ci/travis.sh
 
 .PHONY: doc
 doc: authors readme
 	cargo doc --features 'serde rustc-serialize bincode'
-
diff --git a/ci/core-test/Cargo.toml b/ci/core-test/Cargo.toml
new file mode 100644
index 0000000000..b35ff9a7dc
--- /dev/null
+++ b/ci/core-test/Cargo.toml
@@ -0,0 +1,14 @@
+[package]
+name = "core-test"
+version = "0.1.0"
+authors = [
+    "Kang Seonghoon <public+rust@mearie.org>",
+    "Brandon W Maister <quodlibetor@gmail.com>",
+]
+edition = "2018"
+
+[dependencies]
+chrono = { path = "../..", default-features = false, features = ["serde"] }
+
+[features]
+alloc = ["chrono/alloc"]
\ No newline at end of file
diff --git a/ci/core-test/src/lib.rs b/ci/core-test/src/lib.rs
new file mode 100644
index 0000000000..e311edb5bc
--- /dev/null
+++ b/ci/core-test/src/lib.rs
@@ -0,0 +1,7 @@
+#![no_std]
+
+use chrono::{TimeZone, Utc};
+
+pub fn create_time() {
+    let _ = Utc.ymd(2019, 1, 1).and_hms(0, 0, 0);
+}
diff --git a/ci/travis.sh b/ci/travis.sh
index 98e25ce5ff..18f639bff0 100755
--- a/ci/travis.sh
+++ b/ci/travis.sh
@@ -2,25 +2,50 @@
 
 # This is the script that's executed by travis, you can run it yourself to run
 # the exact same suite
+#
+# When running it locally the most important thing to set is the CHANNEL env
+# var, otherwise it will run tests against every version of rust that it knows
+# about (nightly, beta, stable, 1.13.0):
+#
+#     $ CHANNEL=stable ./ci/travis.sh
 
 set -e
 
 DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
-channel() {
-    if [ -n "${TRAVIS}" ]; then
-        if [ "${TRAVIS_RUST_VERSION}" = "${CHANNEL}" ]; then
-            pwd
-            (set -x; cargo "$@")
-        fi
-    elif [ -n "${APPVEYOR}" ]; then
-        if [ "${APPVEYOR_RUST_CHANNEL}" = "${CHANNEL}" ]; then
-            pwd
-            (set -x; cargo "$@")
+
+main() {
+    if [[ -n "$CHANNEL" ]] ; then
+        if [[ "$CHANNEL" == 1.13.0 ]]; then
+            banner "Building $CHANNEL"
+            build_only
+        else
+            banner "Building/testing $CHANNEL"
+            build_and_test
+            banner "Testing Core $CHANNEL"
+            build_core_test
         fi
     else
-        pwd
-        (set -x; cargo "+${CHANNEL}" "$@")
+        CHANNEL=nightly
+        matching_banner "Test $CHANNEL"
+        if [[ "${CLIPPY}" = y ]] ; then
+            run_clippy
+        else
+            build_and_test
+        fi
+
+        CHANNEL=beta
+        matching_banner "Test $CHANNEL"
+        build_and_test
+
+        CHANNEL=stable
+        matching_banner "Test $CHANNEL"
+        build_and_test
+        build_core_test
+
+        CHANNEL=1.13.0
+        matching_banner "Test $CHANNEL"
+        build_only
     fi
 }
 
@@ -54,14 +79,20 @@ build_and_test_nonwasm() {
   TZ=Asia/Katmandu channel test -v --features serde,rustc-serialize
 
   # without default "clock" feature
-  channel build -v --no-default-features
+  channel build -v --no-default-features --features std
   TZ=ACST-9:30 channel test -v --no-default-features --lib
-  channel build -v --no-default-features --features rustc-serialize
+  channel build -v --no-default-features --features std,rustc-serialize
   TZ=EST4 channel test -v --no-default-features --features rustc-serialize --lib
-  channel build -v --no-default-features --features serde
+  channel build -v --no-default-features --features std,serde
   TZ=UTC0 channel test -v --no-default-features --features serde --lib
-  channel build -v --no-default-features --features serde,rustc-serialize
-  TZ=Asia/Katmandu channel test -v --no-default-features --features serde,rustc-serialize --lib
+  channel build -v --no-default-features --features std,serde,rustc-serialize
+  TZ=Asia/Katmandu channel test -v --no-default-features --features std,serde,rustc-serialize --lib
+
+  channel build -v --no-default-features --features 'serde'
+  TZ=UTC0 channel test -v --no-default-features --features 'serde' --lib
+
+  channel build -v --no-default-features --features 'alloc serde'
+  TZ=UTC0 channel test -v --no-default-features --features 'alloc serde' --lib
 }
 
 build_and_test_wasm() {
@@ -81,8 +112,16 @@ build_only() {
   cargo clean
   channel build -v
   channel build -v --features rustc-serialize
-  channel build -v --features 'serde bincode'
-  channel build -v --no-default-features
+  channel build -v --features serde
+  channel build -v --no-default-features --features std
+}
+
+build_core_test() {
+    channel_run rustup target add thumbv6m-none-eabi --toolchain "$CHANNEL"
+    (
+        cd ci/core-test
+        channel build -v --target thumbv6m-none-eabi
+    )
 }
 
 run_clippy() {
@@ -92,7 +131,7 @@ run_clippy() {
         exit
     fi
 
-    cargo clippy --features 'serde bincode rustc-serialize' -- -Dclippy
+    cargo clippy --features 'serde rustc-serialize' -- -Dclippy
 }
 
 check_readme() {
@@ -100,22 +139,70 @@ check_readme() {
     (set -x; git diff --exit-code -- README.md) ; echo $?
 }
 
-rustc --version
-cargo --version
-node --version
+# script helpers
+
+banner() {
+    echo "======================================================================"
+    echo "$*"
+    echo "======================================================================"
+}
 
-CHANNEL=nightly
-if [ "x${CLIPPY}" = xy ] ; then
-    run_clippy
-else
-    build_and_test
-fi
+underline() {
+    echo "$*"
+    echo "${*//?/^}"
+}
 
-CHANNEL=beta
-build_and_test
+matching_banner() {
+    if channel_matches || ! is_ci ; then
+        banner "$*"
+        echo_versions
+    fi
+}
 
-CHANNEL=stable
-build_and_test
+echo_versions() {
+    channel_run rustc --version
+    channel_run cargo --version
+    node --version
+}
+
+channel() {
+    channel_run cargo "$@"
+}
+
+channel_run() {
+    if channel_matches ; then
+        local the_cmd="$ $*"
+        underline "$the_cmd"
+        "$@"
+    elif ! is_ci ; then
+        local cmd="$1"
+        shift
+        if [[ $cmd == cargo || $cmd == rustc ]] ; then
+            underline "$ $cmd +${CHANNEL} $*"
+            "$cmd" "+${CHANNEL}" "$@"
+        else
+            underline "$ $cmd $*"
+            "$cmd" "$@"
+        fi
+    fi
+}
+
+channel_matches() {
+    if is_ci ; then
+        if [[ "${TRAVIS_RUST_VERSION}" = "${CHANNEL}"
+              || "${APPVEYOR_RUST_CHANNEL}" = "${CHANNEL}" ]] ; then
+            return 0
+        fi
+    fi
+    return 1
+}
+
+is_ci() {
+    if [[ -n "$TRAVIS" || -n "$APPVEYOR" ]] ; then
+        return 0
+    else
+        return 1
+    fi
+}
 
-CHANNEL=1.13.0
-build_only
+main
diff --git a/src/date.rs b/src/date.rs
index f8b59cea77..d4490092d4 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -3,16 +3,17 @@
 
 //! ISO 8601 calendar date with time zone.
 
-use std::{fmt, hash};
-use std::cmp::Ordering;
-use std::ops::{Add, Sub};
+use core::{fmt, hash};
+use core::cmp::Ordering;
+use core::ops::{Add, Sub};
 use oldtime::Duration as OldDuration;
 
 use {Weekday, Datelike};
 use offset::{TimeZone, Utc};
 use naive::{self, NaiveDate, NaiveTime, IsoWeek};
 use DateTime;
-use format::{Item, DelayedFormat, StrftimeItems};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use format::{DelayedFormat, Item, StrftimeItems};
 
 /// ISO 8601 calendar date with time zone.
 ///
@@ -255,6 +256,7 @@ fn map_local<Tz: TimeZone, F>(d: &Date<Tz>, mut f: F) -> Option<Date<Tz>>
 
 impl<Tz: TimeZone> Date<Tz> where Tz::Offset: fmt::Display {
     /// Formats the date with the specified formatting items.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format_with_items<'a, I>(&self, items: I) -> DelayedFormat<I>
             where I: Iterator<Item=Item<'a>> + Clone {
@@ -264,6 +266,7 @@ impl<Tz: TimeZone> Date<Tz> where Tz::Offset: fmt::Display {
     /// Formats the date with the specified format string.
     /// See the [`format::strftime` module](./format/strftime/index.html)
     /// on the supported escape sequences.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format<'a>(&self, fmt: &'a str) -> DelayedFormat<StrftimeItems<'a>> {
         self.format_with_items(StrftimeItems::new(fmt))
diff --git a/src/datetime.rs b/src/datetime.rs
index b4881bf401..aa61fe4913 100644
--- a/src/datetime.rs
+++ b/src/datetime.rs
@@ -3,12 +3,18 @@
 
 //! ISO 8601 date and time with time zone.
 
-use std::{str, fmt, hash};
-use std::cmp::Ordering;
-use std::ops::{Add, Sub};
+use core::{str, fmt, hash};
+use core::cmp::Ordering;
+use core::ops::{Add, Sub};
+#[cfg(any(feature = "std", test))]
 use std::time::{SystemTime, UNIX_EPOCH};
 use oldtime::Duration as OldDuration;
 
+#[cfg(all(not(feature = "std"), feature = "alloc"))]
+use alloc::string::{String, ToString};
+#[cfg(feature = "std")]
+use std::string::ToString;
+
 use {Weekday, Timelike, Datelike};
 #[cfg(feature="clock")]
 use offset::Local;
@@ -16,7 +22,9 @@ use offset::{TimeZone, Offset, Utc, FixedOffset};
 use naive::{NaiveTime, NaiveDateTime, IsoWeek};
 use Date;
 use format::{Item, Numeric, Pad, Fixed};
-use format::{parse, Parsed, ParseError, ParseResult, DelayedFormat, StrftimeItems};
+use format::{parse, Parsed, ParseError, ParseResult, StrftimeItems};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use format::DelayedFormat;
 
 /// Specific formatting options for seconds. This may be extended in the
 /// future, so exhaustive matching in external code is not recommended.
@@ -363,12 +371,14 @@ impl DateTime<FixedOffset> {
 
 impl<Tz: TimeZone> DateTime<Tz> where Tz::Offset: fmt::Display {
     /// Returns an RFC 2822 date and time string such as `Tue, 1 Jul 2003 10:52:37 +0200`.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     pub fn to_rfc2822(&self) -> String {
         const ITEMS: &'static [Item<'static>] = &[Item::Fixed(Fixed::RFC2822)];
         self.format_with_items(ITEMS.iter().cloned()).to_string()
     }
 
     /// Returns an RFC 3339 and ISO 8601 date and time string such as `1996-12-19T16:39:57-08:00`.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     pub fn to_rfc3339(&self) -> String {
         const ITEMS: &'static [Item<'static>] = &[Item::Fixed(Fixed::RFC3339)];
         self.format_with_items(ITEMS.iter().cloned()).to_string()
@@ -398,6 +408,7 @@ impl<Tz: TimeZone> DateTime<Tz> where Tz::Offset: fmt::Display {
     /// assert_eq!(dt.to_rfc3339_opts(SecondsFormat::Secs, true),
     ///            "2018-01-26T10:30:09+08:00");
     /// ```
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     pub fn to_rfc3339_opts(&self, secform: SecondsFormat, use_z: bool) -> String {
         use format::Numeric::*;
         use format::Pad::Zero;
@@ -449,6 +460,7 @@ impl<Tz: TimeZone> DateTime<Tz> where Tz::Offset: fmt::Display {
     }
 
     /// Formats the combined date and time with the specified formatting items.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format_with_items<'a, I>(&self, items: I) -> DelayedFormat<I>
             where I: Iterator<Item=Item<'a>> + Clone {
@@ -459,6 +471,7 @@ impl<Tz: TimeZone> DateTime<Tz> where Tz::Offset: fmt::Display {
     /// Formats the combined date and time with the specified format string.
     /// See the [`format::strftime` module](./format/strftime/index.html)
     /// on the supported escape sequences.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format<'a>(&self, fmt: &'a str) -> DelayedFormat<StrftimeItems<'a>> {
         self.format_with_items(StrftimeItems::new(fmt))
@@ -647,6 +660,7 @@ impl str::FromStr for DateTime<Local> {
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl From<SystemTime> for DateTime<Utc> {
     fn from(t: SystemTime) -> DateTime<Utc> {
         let (sec, nsec) = match t.duration_since(UNIX_EPOCH) {
@@ -672,6 +686,7 @@ impl From<SystemTime> for DateTime<Local> {
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl<Tz: TimeZone> From<DateTime<Tz>> for SystemTime {
     fn from(dt: DateTime<Tz>) -> SystemTime {
         use std::time::Duration;
@@ -699,7 +714,7 @@ fn test_auto_conversion() {
 fn test_encodable_json<FUtc, FFixed, E>(to_string_utc: FUtc, to_string_fixed: FFixed)
     where FUtc: Fn(&DateTime<Utc>) -> Result<String, E>,
           FFixed: Fn(&DateTime<FixedOffset>) -> Result<String, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     assert_eq!(to_string_utc(&Utc.ymd(2014, 7, 24).and_hms(12, 34, 6)).ok(),
                Some(r#""2014-07-24T12:34:06Z""#.into()));
@@ -717,7 +732,7 @@ fn test_decodable_json<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
     where FUtc: Fn(&str) -> Result<DateTime<Utc>, E>,
           FFixed: Fn(&str) -> Result<DateTime<FixedOffset>, E>,
           FLocal: Fn(&str) -> Result<DateTime<Local>, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     // should check against the offset as well (the normal DateTime comparison will ignore them)
     fn norm<Tz: TimeZone>(dt: &Option<DateTime<Tz>>) -> Option<(&DateTime<Tz>, &Tz::Offset)> {
@@ -754,7 +769,7 @@ fn test_decodable_json_timestamps<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
     where FUtc: Fn(&str) -> Result<rustc_serialize::TsSeconds<Utc>, E>,
           FFixed: Fn(&str) -> Result<rustc_serialize::TsSeconds<FixedOffset>, E>,
           FLocal: Fn(&str) -> Result<rustc_serialize::TsSeconds<Local>, E>,
-          E: ::std::fmt::Debug
+          E: ::core::fmt::Debug
 {
     fn norm<Tz: TimeZone>(dt: &Option<DateTime<Tz>>) -> Option<(&DateTime<Tz>, &Tz::Offset)> {
         dt.as_ref().map(|dt| (dt, dt.offset()))
@@ -778,8 +793,8 @@ fn test_decodable_json_timestamps<FUtc, FFixed, FLocal, E>(utc_from_str: FUtc,
 
 #[cfg(feature = "rustc-serialize")]
 pub mod rustc_serialize {
-    use std::fmt;
-    use std::ops::Deref;
+    use core::fmt;
+    use core::ops::Deref;
     use super::DateTime;
     #[cfg(feature="clock")]
     use offset::Local;
@@ -907,25 +922,26 @@ pub mod rustc_serialize {
 /// documented at re-export site
 #[cfg(feature = "serde")]
 pub mod serde {
-    use std::fmt;
+    use core::fmt;
     use super::DateTime;
     #[cfg(feature="clock")]
     use offset::Local;
     use offset::{LocalResult, TimeZone, Utc, FixedOffset};
     use serdelib::{ser, de};
+    use {SerdeError, ne_timestamp};
 
     // try!-like function to convert a LocalResult into a serde-ish Result
     fn serde_from<T, E, V>(me: LocalResult<T>, ts: &V) -> Result<T, E>
-        where E: de::Error,
-                V: fmt::Display,
-                T: fmt::Display,
+    where
+        E: de::Error,
+        V: fmt::Display,
+        T: fmt::Display,
     {
         match me {
             LocalResult::None => Err(E::custom(
-                format!("value is not a legal timestamp: {}", ts))),
+                ne_timestamp(ts))),
             LocalResult::Ambiguous(min, max) => Err(E::custom(
-                format!("value is an ambiguous timestamp: {}, could be either of {}, {}",
-                        ts, min, max))),
+                SerdeError::Ambiguous { timestamp: ts, min: min, max: max })),
             LocalResult::Single(val) => Ok(val)
         }
     }
@@ -967,7 +983,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_nanoseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
@@ -1114,7 +1130,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_milliseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
@@ -1261,7 +1277,7 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_seconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
         use {DateTime, Utc};
@@ -1403,7 +1419,7 @@ pub mod serde {
         fn visit_str<E>(self, value: &str) -> Result<DateTime<FixedOffset>, E>
             where E: de::Error
         {
-            value.parse().map_err(|err| E::custom(format!("{}", err)))
+            value.parse().map_err(|err: ::format::ParseError| E::custom(err))
         }
     }
 
diff --git a/src/format/mod.rs b/src/format/mod.rs
index 24647b4112..b8e6458345 100644
--- a/src/format/mod.rs
+++ b/src/format/mod.rs
@@ -17,20 +17,30 @@
 
 #![allow(ellipsis_inclusive_range_patterns)]
 
-use std::fmt;
-use std::str::FromStr;
+use core::fmt;
+use core::str::FromStr;
+#[cfg(any(feature = "std", test))]
 use std::error::Error;
-
-use {Datelike, Timelike, Weekday, ParseWeekdayError};
+#[cfg(feature = "alloc")]
+use alloc::boxed::Box;
+#[cfg(feature = "alloc")]
+use alloc::string::{String, ToString};
+
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use {Datelike, Timelike};
+use {Weekday, ParseWeekdayError};
+#[cfg(any(feature = "alloc", feature = "std", test))]
 use div::{div_floor, mod_floor};
+#[cfg(any(feature = "alloc", feature = "std", test))]
 use offset::{Offset, FixedOffset};
+#[cfg(any(feature = "alloc", feature = "std", test))]
 use naive::{NaiveDate, NaiveTime};
 
 pub use self::strftime::StrftimeItems;
 pub use self::parsed::Parsed;
 pub use self::parse::parse;
 
-/// An unhabitated type used for `InternalNumeric` and `InternalFixed` below.
+/// An uninhabited type used for `InternalNumeric` and `InternalFixed` below.
 #[derive(Clone, PartialEq, Eq)]
 enum Void {}
 
@@ -55,7 +65,7 @@ pub enum Pad {
 ///
 /// The **parsing width** is the maximal width to be scanned.
 /// The parser only tries to consume from one to given number of digits (greedily).
-/// It also trims the preceding whitespaces if any.
+/// It also trims the preceding whitespace if any.
 /// It cannot parse the negative number, so some date and time cannot be formatted then
 /// parsed with the same formatting items.
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -185,13 +195,13 @@ pub enum Fixed {
     TimezoneName,
     /// Offset from the local time to UTC (`+09:00` or `-04:00` or `+00:00`).
     ///
-    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespaces.
+    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespace.
     /// The offset is limited from `-24:00` to `+24:00`,
     /// which is same to [`FixedOffset`](../offset/struct.FixedOffset.html)'s range.
     TimezoneOffsetColon,
     /// Offset from the local time to UTC (`+09:00` or `-04:00` or `Z`).
     ///
-    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespaces,
+    /// In the parser, the colon can be omitted and/or surrounded with any amount of whitespace,
     /// and `Z` can be either in upper case or in lower case.
     /// The offset is limited from `-24:00` to `+24:00`,
     /// which is same to [`FixedOffset`](../offset/struct.FixedOffset.html)'s range.
@@ -245,10 +255,12 @@ pub enum Item<'a> {
     /// A literally printed and parsed text.
     Literal(&'a str),
     /// Same to `Literal` but with the string owned by the item.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     OwnedLiteral(Box<str>),
     /// Whitespace. Prints literally but reads zero or more whitespace.
     Space(&'a str),
     /// Same to `Space` but with the string owned by the item.
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     OwnedSpace(Box<str>),
     /// Numeric item. Can be optionally padded to the maximal length (if any) when formatting;
     /// the parser simply ignores any padded whitespace and zeroes.
@@ -305,13 +317,7 @@ enum ParseErrorKind {
 /// Same to `Result<T, ParseError>`.
 pub type ParseResult<T> = Result<T, ParseError>;
 
-impl fmt::Display for ParseError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        self.description().fmt(f)
-    }
-}
-
-impl Error for ParseError {
+impl ParseError {
     fn description(&self) -> &str {
         match self.0 {
             ParseErrorKind::OutOfRange => "input is out of range",
@@ -325,6 +331,19 @@ impl Error for ParseError {
     }
 }
 
+impl fmt::Display for ParseError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.description().fmt(f)
+    }
+}
+
+#[cfg(any(feature = "std", test))]
+impl Error for ParseError {
+    fn description(&self) -> &str {
+        self.description()
+    }
+}
+
 // to be used in this module and submodules
 const OUT_OF_RANGE: ParseError = ParseError(ParseErrorKind::OutOfRange);
 const IMPOSSIBLE:   ParseError = ParseError(ParseErrorKind::Impossible);
@@ -336,6 +355,7 @@ const BAD_FORMAT:   ParseError = ParseError(ParseErrorKind::BadFormat);
 
 /// Tries to format given arguments with given formatting items.
 /// Internally used by `DelayedFormat`.
+#[cfg(any(feature = "alloc", feature = "std", test))]
 pub fn format<'a, I>(
     w: &mut fmt::Formatter,
     date: Option<&NaiveDate>,
@@ -356,12 +376,13 @@ pub fn format<'a, I>(
     static LONG_WEEKDAYS: [&'static str; 7] =
         ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"];
 
-    use std::fmt::Write;
+    use core::fmt::Write;
     let mut result = String::new();
 
     for item in items {
         match item {
             Item::Literal(s) | Item::Space(s) => result.push_str(s),
+            #[cfg(any(feature = "alloc", feature = "std", test))]
             Item::OwnedLiteral(ref s) | Item::OwnedSpace(ref s) => result.push_str(s),
 
             Item::Numeric(spec, pad) => {
@@ -598,6 +619,7 @@ pub mod strftime;
 
 /// A *temporary* object which can be used as an argument to `format!` or others.
 /// This is normally constructed via `format` methods of each date and time type.
+#[cfg(any(feature = "alloc", feature = "std", test))]
 #[derive(Debug)]
 pub struct DelayedFormat<I> {
     /// The date view, if any.
@@ -610,6 +632,7 @@ pub struct DelayedFormat<I> {
     items: I,
 }
 
+#[cfg(any(feature = "alloc", feature = "std", test))]
 impl<'a, I: Iterator<Item=Item<'a>> + Clone> DelayedFormat<I> {
     /// Makes a new `DelayedFormat` value out of local date and time.
     pub fn new(date: Option<NaiveDate>, time: Option<NaiveTime>, items: I) -> DelayedFormat<I> {
@@ -625,6 +648,7 @@ impl<'a, I: Iterator<Item=Item<'a>> + Clone> DelayedFormat<I> {
     }
 }
 
+#[cfg(any(feature = "alloc", feature = "std", test))]
 impl<'a, I: Iterator<Item=Item<'a>> + Clone> fmt::Display for DelayedFormat<I> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         format(f, self.date.as_ref(), self.time.as_ref(), self.off.as_ref(), self.items.clone())
diff --git a/src/format/parse.rs b/src/format/parse.rs
index 01e326eece..e8e0d25752 100644
--- a/src/format/parse.rs
+++ b/src/format/parse.rs
@@ -6,7 +6,7 @@
 
 #![allow(deprecated)]
 
-use std::usize;
+use core::usize;
 
 use Weekday;
 
@@ -218,13 +218,19 @@ pub fn parse<'a, I>(parsed: &mut Parsed, mut s: &str, items: I) -> ParseResult<(
                 s = &s[prefix.len()..];
             }
 
+            #[cfg(any(feature = "alloc", feature = "std", test))]
             Item::OwnedLiteral(ref prefix) => {
                 if s.len() < prefix.len() { return Err(TOO_SHORT); }
                 if !s.starts_with(&prefix[..]) { return Err(INVALID); }
                 s = &s[prefix.len()..];
             }
 
-            Item::Space(_) | Item::OwnedSpace(_) => {
+            Item::Space(_) => {
+                s = s.trim_left();
+            }
+
+            #[cfg(any(feature = "alloc", feature = "std", test))]
+            Item::OwnedSpace(_) => {
                 s = s.trim_left();
             }
 
diff --git a/src/lib.rs b/src/lib.rs
index b43fa0dcf8..f9db6f7c47 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -387,6 +387,8 @@
 #![deny(missing_docs)]
 #![deny(missing_debug_implementations)]
 
+#![cfg_attr(not(any(feature = "std", test)), no_std)]
+
 // The explicit 'static lifetimes are still needed for rustc 1.13-16
 // backward compatibility, and this appeases clippy. If minimum rustc
 // becomes 1.17, should be able to remove this, those 'static lifetimes,
@@ -403,6 +405,13 @@
     trivially_copy_pass_by_ref,
 ))]
 
+#[cfg(feature = "alloc")]
+extern crate alloc;
+#[cfg(any(feature = "std", test))]
+extern crate std as core;
+#[cfg(all(feature = "std", not(feature="alloc")))]
+extern crate std as alloc;
+
 #[cfg(feature="clock")]
 extern crate time as oldtime;
 extern crate num_integer;
@@ -511,6 +520,41 @@ pub mod serde {
     pub use super::datetime::serde::*;
 }
 
+// Until rust 1.18 there  is no "pub(crate)" so to share this we need it in the root
+
+#[cfg(feature = "serde")]
+enum SerdeError<V: fmt::Display, D: fmt::Display> {
+    NonExistent { timestamp: V },
+    Ambiguous { timestamp: V, min: D, max: D },
+}
+
+/// Construct a [`SerdeError::NonExistent`]
+#[cfg(feature = "serde")]
+fn ne_timestamp<T: fmt::Display>(ts: T) -> SerdeError<T, u8> {
+    SerdeError::NonExistent::<T, u8> { timestamp: ts }
+}
+
+#[cfg(feature = "serde")]
+impl<V: fmt::Display, D: fmt::Display> fmt::Debug for SerdeError<V, D> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "ChronoSerdeError({})", self)
+    }
+}
+
+// impl<V: fmt::Display, D: fmt::Debug> core::error::Error for SerdeError<V, D> {}
+#[cfg(feature = "serde")]
+impl<V: fmt::Display, D: fmt::Display> fmt::Display for SerdeError<V, D> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            &SerdeError::NonExistent { ref timestamp } => write!(
+                f, "value is not a legal timestamp: {}", timestamp),
+            &SerdeError::Ambiguous { ref timestamp, ref min, ref max } => write!(
+                f, "value is an ambiguous timestamp: {}, could be either of {}, {}",
+                timestamp, min, max),
+        }
+    }
+}
+
 /// The day of week.
 ///
 /// The order of the days of week depends on the context.
@@ -645,6 +689,20 @@ impl Weekday {
     }
 }
 
+impl fmt::Display for Weekday {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(match *self {
+            Weekday::Mon => "Mon",
+            Weekday::Tue => "Tue",
+            Weekday::Wed => "Wed",
+            Weekday::Thu => "Thu",
+            Weekday::Fri => "Fri",
+            Weekday::Sat => "Sat",
+            Weekday::Sun => "Sun",
+        })
+    }
+}
+
 /// Any weekday can be represented as an integer from 0 to 6, which equals to
 /// [`Weekday::num_days_from_monday`](#method.num_days_from_monday) in this implementation.
 /// Do not heavily depend on this though; use explicit methods whenever possible.
@@ -678,7 +736,7 @@ impl num_traits::FromPrimitive for Weekday {
     }
 }
 
-use std::fmt;
+use core::fmt;
 
 /// An error resulting from reading `Weekday` value with `FromStr`.
 #[derive(Clone, PartialEq)]
@@ -697,14 +755,14 @@ impl fmt::Debug for ParseWeekdayError {
 #[cfg(feature = "serde")]
 mod weekday_serde {
     use super::Weekday;
-    use std::fmt;
+    use core::fmt;
     use serdelib::{ser, de};
 
     impl ser::Serialize for Weekday {
         fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
             where S: ser::Serializer
         {
-            serializer.serialize_str(&format!("{:?}", self))
+            serializer.collect_str(&self)
         }
     }
 
diff --git a/src/naive/date.rs b/src/naive/date.rs
index aad98bc605..981e9f4bf1 100644
--- a/src/naive/date.rs
+++ b/src/naive/date.rs
@@ -3,8 +3,8 @@
 
 //! ISO 8601 calendar date without timezone.
 
-use std::{str, fmt};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use num_traits::ToPrimitive;
 use oldtime::Duration as OldDuration;
 
@@ -12,7 +12,9 @@ use {Weekday, Datelike};
 use div::div_mod_floor;
 use naive::{NaiveTime, NaiveDateTime, IsoWeek};
 use format::{Item, Numeric, Pad};
-use format::{parse, Parsed, ParseError, ParseResult, DelayedFormat, StrftimeItems};
+use format::{parse, Parsed, ParseError, ParseResult, StrftimeItems};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use format::DelayedFormat;
 
 use super::isoweek;
 use super::internals::{self, DateImpl, Of, Mdf, YearFlags};
@@ -916,6 +918,7 @@ impl NaiveDate {
     /// # let d = NaiveDate::from_ymd(2015, 9, 5);
     /// assert_eq!(format!("{}", d.format_with_items(fmt)), "2015-09-05");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format_with_items<'a, I>(&self, items: I) -> DelayedFormat<I>
             where I: Iterator<Item=Item<'a>> + Clone {
@@ -954,6 +957,7 @@ impl NaiveDate {
     /// assert_eq!(format!("{}", d.format("%Y-%m-%d")), "2015-09-05");
     /// assert_eq!(format!("{}", d.format("%A, %-d %B, %C%y")), "Saturday, 5 September, 2015");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format<'a>(&self, fmt: &'a str) -> DelayedFormat<StrftimeItems<'a>> {
         self.format_with_items(StrftimeItems::new(fmt))
@@ -1387,7 +1391,7 @@ impl SubAssign<OldDuration> for NaiveDate {
 
 /// Subtracts another `NaiveDate` from the current date.
 /// Returns a `Duration` of integral numbers.
-/// 
+///
 /// This does not overflow or underflow at all,
 /// as all possible output fits in the range of `Duration`.
 ///
@@ -1600,7 +1604,7 @@ mod rustc_serialize {
 
 #[cfg(feature = "serde")]
 mod serde {
-    use std::fmt;
+    use core::fmt;
     use super::NaiveDate;
     use serdelib::{ser, de};
 
@@ -1629,15 +1633,23 @@ mod serde {
     impl<'de> de::Visitor<'de> for NaiveDateVisitor {
         type Value = NaiveDate;
 
-        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result 
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result
         {
             write!(formatter, "a formatted date string")
         }
 
+        #[cfg(any(feature = "std", test))]
+        fn visit_str<E>(self, value: &str) -> Result<NaiveDate, E>
+            where E: de::Error
+        {
+            value.parse().map_err(E::custom)
+        }
+
+        #[cfg(not(any(feature = "std", test)))]
         fn visit_str<E>(self, value: &str) -> Result<NaiveDate, E>
             where E: de::Error
         {
-            value.parse().map_err(|err| E::custom(format!("{}", err)))
+            value.parse().map_err(E::custom)
         }
     }
 
diff --git a/src/naive/datetime.rs b/src/naive/datetime.rs
index bb9ff8ddf4..2049782203 100644
--- a/src/naive/datetime.rs
+++ b/src/naive/datetime.rs
@@ -3,8 +3,8 @@
 
 //! ISO 8601 date and time without timezone.
 
-use std::{str, fmt, hash};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt, hash};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use num_traits::ToPrimitive;
 use oldtime::Duration as OldDuration;
 
@@ -12,7 +12,9 @@ use {Weekday, Timelike, Datelike};
 use div::div_mod_floor;
 use naive::{NaiveTime, NaiveDate, IsoWeek};
 use format::{Item, Numeric, Pad, Fixed};
-use format::{parse, Parsed, ParseError, ParseResult, DelayedFormat, StrftimeItems};
+use format::{parse, Parsed, ParseError, ParseResult, StrftimeItems};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use format::DelayedFormat;
 
 /// The tight upper bound guarantees that a duration with `|Duration| >= 2^MAX_SECS_BITS`
 /// will always overflow the addition with any date and time type.
@@ -645,6 +647,7 @@ impl NaiveDateTime {
     /// # let dt = NaiveDate::from_ymd(2015, 9, 5).and_hms(23, 56, 4);
     /// assert_eq!(format!("{}", dt.format_with_items(fmt)), "2015-09-05 23:56:04");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format_with_items<'a, I>(&self, items: I) -> DelayedFormat<I>
             where I: Iterator<Item=Item<'a>> + Clone {
@@ -683,6 +686,7 @@ impl NaiveDateTime {
     /// assert_eq!(format!("{}", dt.format("%Y-%m-%d %H:%M:%S")), "2015-09-05 23:56:04");
     /// assert_eq!(format!("{}", dt.format("around %l %p on %b %-d")), "around 11 PM on Sep 5");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format<'a>(&self, fmt: &'a str) -> DelayedFormat<StrftimeItems<'a>> {
         self.format_with_items(StrftimeItems::new(fmt))
@@ -1663,7 +1667,7 @@ pub mod rustc_serialize {
 /// Tools to help serializing/deserializing `NaiveDateTime`s
 #[cfg(feature = "serde")]
 pub mod serde {
-    use std::fmt;
+    use core::fmt;
     use super::{NaiveDateTime};
     use serdelib::{ser, de};
 
@@ -1702,7 +1706,7 @@ pub mod serde {
         fn visit_str<E>(self, value: &str) -> Result<NaiveDateTime, E>
             where E: de::Error
         {
-            value.parse().map_err(|err| E::custom(format!("{}", err)))
+            value.parse().map_err(E::custom)
         }
     }
 
@@ -1750,10 +1754,10 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_nanoseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
-        use NaiveDateTime;
+        use {NaiveDateTime, ne_timestamp};
 
         /// Serialize a UTC datetime into an integer number of nanoseconds since the epoch
         ///
@@ -1846,7 +1850,7 @@ pub mod serde {
             {
                 NaiveDateTime::from_timestamp_opt(value / 1_000_000_000,
                                                  (value % 1_000_000_000) as u32)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
 
             fn visit_u64<E>(self, value: u64) -> Result<NaiveDateTime, E>
@@ -1854,7 +1858,7 @@ pub mod serde {
             {
                 NaiveDateTime::from_timestamp_opt(value as i64 / 1_000_000_000,
                                                  (value as i64 % 1_000_000_000) as u32)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
         }
     }
@@ -1895,10 +1899,10 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_milliseconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
-        use NaiveDateTime;
+        use {NaiveDateTime, ne_timestamp};
 
         /// Serialize a UTC datetime into an integer number of milliseconds since the epoch
         ///
@@ -1991,7 +1995,7 @@ pub mod serde {
             {
                 NaiveDateTime::from_timestamp_opt(value / 1000,
                                                 ((value % 1000) * 1_000_000) as u32)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
 
             fn visit_u64<E>(self, value: u64) -> Result<NaiveDateTime, E>
@@ -1999,7 +2003,7 @@ pub mod serde {
             {
                 NaiveDateTime::from_timestamp_opt((value / 1000) as i64,
                                                  ((value % 1000) * 1_000_000) as u32)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
         }
     }
@@ -2040,10 +2044,10 @@ pub mod serde {
     /// # fn main() { example().unwrap(); }
     /// ```
     pub mod ts_seconds {
-        use std::fmt;
+        use core::fmt;
         use serdelib::{ser, de};
 
-        use NaiveDateTime;
+        use {NaiveDateTime, ne_timestamp};
 
         /// Serialize a UTC datetime into an integer number of seconds since the epoch
         ///
@@ -2135,14 +2139,14 @@ pub mod serde {
                 where E: de::Error
             {
                 NaiveDateTime::from_timestamp_opt(value, 0)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
 
             fn visit_u64<E>(self, value: u64) -> Result<NaiveDateTime, E>
                 where E: de::Error
             {
                 NaiveDateTime::from_timestamp_opt(value as i64, 0)
-                    .ok_or_else(|| E::custom(format!("value is not a legal timestamp: {}", value)))
+                    .ok_or_else(|| E::custom(ne_timestamp(value)))
             }
         }
     }
diff --git a/src/naive/internals.rs b/src/naive/internals.rs
index dd9d535b1b..a1cd1f6d46 100644
--- a/src/naive/internals.rs
+++ b/src/naive/internals.rs
@@ -15,7 +15,7 @@
 
 #![allow(dead_code)] // some internal methods have been left for consistency
 
-use std::{i32, fmt};
+use core::{i32, fmt};
 use num_traits::FromPrimitive;
 use Weekday;
 use div::{div_rem, mod_floor};
diff --git a/src/naive/isoweek.rs b/src/naive/isoweek.rs
index 667cf2f8e9..0aeedb0139 100644
--- a/src/naive/isoweek.rs
+++ b/src/naive/isoweek.rs
@@ -3,7 +3,7 @@
 
 //! ISO 8601 week.
 
-use std::fmt;
+use core::fmt;
 
 use super::internals::{DateImpl, Of, YearFlags};
 
diff --git a/src/naive/time.rs b/src/naive/time.rs
index 440c8a7ac0..c0272c8fe9 100644
--- a/src/naive/time.rs
+++ b/src/naive/time.rs
@@ -3,14 +3,16 @@
 
 //! ISO 8601 time without timezone.
 
-use std::{str, fmt, hash};
-use std::ops::{Add, Sub, AddAssign, SubAssign};
+use core::{str, fmt, hash};
+use core::ops::{Add, Sub, AddAssign, SubAssign};
 use oldtime::Duration as OldDuration;
 
 use Timelike;
 use div::div_mod_floor;
 use format::{Item, Numeric, Pad, Fixed};
-use format::{parse, Parsed, ParseError, ParseResult, DelayedFormat, StrftimeItems};
+use format::{parse, Parsed, ParseError, ParseResult, StrftimeItems};
+#[cfg(any(feature = "alloc", feature = "std", test))]
+use format::DelayedFormat;
 
 /// ISO 8601 time without timezone.
 /// Allows for the nanosecond precision and optional leap second representation.
@@ -681,7 +683,7 @@ impl NaiveTime {
         //      `rhs.frac`|========================================>|
         //          |     |   |        `self - rhs`         |       |
 
-        use std::cmp::Ordering;
+        use core::cmp::Ordering;
 
         let secs = i64::from(self.secs) - i64::from(rhs.secs);
         let frac = i64::from(self.frac) - i64::from(rhs.frac);
@@ -723,6 +725,7 @@ impl NaiveTime {
     /// # let t = NaiveTime::from_hms(23, 56, 4);
     /// assert_eq!(format!("{}", t.format_with_items(fmt)), "23:56:04");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format_with_items<'a, I>(&self, items: I) -> DelayedFormat<I>
             where I: Iterator<Item=Item<'a>> + Clone {
@@ -763,6 +766,7 @@ impl NaiveTime {
     /// assert_eq!(format!("{}", t.format("%H:%M:%S%.6f")), "23:56:04.012345");
     /// assert_eq!(format!("{}", t.format("%-I:%M %p")), "11:56 PM");
     /// ~~~~
+    #[cfg(any(feature = "alloc", feature = "std", test))]
     #[inline]
     pub fn format<'a>(&self, fmt: &'a str) -> DelayedFormat<StrftimeItems<'a>> {
         self.format_with_items(StrftimeItems::new(fmt))
@@ -1411,7 +1415,7 @@ mod rustc_serialize {
 
 #[cfg(feature = "serde")]
 mod serde {
-    use std::fmt;
+    use core::fmt;
     use super::NaiveTime;
     use serdelib::{ser, de};
 
@@ -1431,7 +1435,7 @@ mod serde {
     impl<'de> de::Visitor<'de> for NaiveTimeVisitor {
         type Value = NaiveTime;
 
-        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result 
+        fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result
         {
             write!(formatter, "a formatted time string")
         }
@@ -1439,7 +1443,7 @@ mod serde {
         fn visit_str<E>(self, value: &str) -> Result<NaiveTime, E>
             where E: de::Error
         {
-            value.parse().map_err(|err| E::custom(format!("{}", err)))
+            value.parse().map_err(E::custom)
         }
     }
 
diff --git a/src/offset/fixed.rs b/src/offset/fixed.rs
index 5b69ef06cd..01512c0124 100644
--- a/src/offset/fixed.rs
+++ b/src/offset/fixed.rs
@@ -3,8 +3,8 @@
 
 //! The time zone which has a fixed offset from UTC.
 
-use std::ops::{Add, Sub};
-use std::fmt;
+use core::ops::{Add, Sub};
+use core::fmt;
 use oldtime::Duration as OldDuration;
 
 use Timelike;
diff --git a/src/offset/mod.rs b/src/offset/mod.rs
index 566e427a97..a25529287c 100644
--- a/src/offset/mod.rs
+++ b/src/offset/mod.rs
@@ -18,7 +18,7 @@
 //! and provides implementations for 1 and 3.
 //! An `TimeZone` instance can be reconstructed from the corresponding `Offset` instance.
 
-use std::fmt;
+use core::fmt;
 
 use format::{parse, ParseResult, Parsed, StrftimeItems};
 use naive::{NaiveDate, NaiveDateTime, NaiveTime};
diff --git a/src/offset/utc.rs b/src/offset/utc.rs
index 87a4fbaf77..da8de11ffa 100644
--- a/src/offset/utc.rs
+++ b/src/offset/utc.rs
@@ -3,7 +3,7 @@
 
 //! The UTC (Coordinated Universal Time) time zone.
 
-use std::fmt;
+use core::fmt;
 #[cfg(all(feature="clock", not(all(target_arch = "wasm32", feature = "wasmbind"))))]
 use oldtime;
 
diff --git a/src/oldtime.rs b/src/oldtime.rs
index 2bdc2f64ef..d523b54eab 100644
--- a/src/oldtime.rs
+++ b/src/oldtime.rs
@@ -10,10 +10,11 @@
 
 //! Temporal quantification
 
-use std::{fmt, i64};
+use core::{fmt, i64};
+#[cfg(any(feature = "std", test))]
 use std::error::Error;
-use std::ops::{Add, Sub, Mul, Div, Neg};
-use std::time::Duration as StdDuration;
+use core::ops::{Add, Sub, Mul, Div, Neg};
+use core::time::Duration as StdDuration;
 
 /// The number of nanoseconds in a microsecond.
 const NANOS_PER_MICRO: i32 = 1000;
@@ -392,15 +393,22 @@ impl fmt::Display for Duration {
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct OutOfRangeError(());
 
+impl OutOfRangeError {
+    fn description(&self) -> &str {
+        "Source duration value is out of range for the target type"
+    }
+}
+
 impl fmt::Display for OutOfRangeError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{}", self.description())
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl Error for OutOfRangeError {
     fn description(&self) -> &str {
-        "Source duration value is out of range for the target type"
+        self.description()
     }
 }
 
diff --git a/src/round.rs b/src/round.rs
index bf62762165..ac5b984be3 100644
--- a/src/round.rs
+++ b/src/round.rs
@@ -2,7 +2,7 @@
 // See README.md and LICENSE.txt for details.
 
 use Timelike;
-use std::ops::{Add, Sub};
+use core::ops::{Add, Sub};
 use oldtime::Duration;
 
 /// Extension trait for subsecond rounding or truncation to a maximum number