diff --git a/modules/core/src-js/smithy4s/Timestamp.scala b/modules/core/src-js/smithy4s/Timestamp.scala
index 28772fcf2..c34dd2083 100644
--- a/modules/core/src-js/smithy4s/Timestamp.scala
+++ b/modules/core/src-js/smithy4s/Timestamp.scala
@@ -17,7 +17,6 @@
 package smithy4s
 
 import smithy.api.TimestampFormat
-import smithy4s.Timestamp._
 import scalajs.js.Date
 import scala.util.control.{NoStackTrace, NonFatal}
 
@@ -77,9 +76,10 @@ case class Timestamp private (epochSecond: Long, nano: Int) {
       marchDayOfYear - ((marchMonth * 1002762 - 16383) >> 15) // marchDayOfYear - (marchMonth * 306 + 5) / 10 + 1
     internalFormat match {
       case 1 =>
-        s.append(daysOfWeek(((epochDay + 700000003) % 7).toInt)).append(',')
+        s.append(Timestamp.daysOfWeek(((epochDay + 700000003) % 7).toInt))
+          .append(',')
         append2Digits(day, s.append(' '))
-        s.append(' ').append(months(month - 1))
+        s.append(' ').append(Timestamp.months(month - 1))
         append4Digits(year, s.append(' '))
         appendTime(secsOfDay, s.append(' '), addSeparator = true)
         appendNano(nano, s)
@@ -141,7 +141,7 @@ case class Timestamp private (epochSecond: Long, nano: Int) {
       append2Digits(q1, s)
       val q2 = r1 / 100000
       val r2 = r1 - q2 * 100000
-      val d = digits(q2)
+      val d = Timestamp.digits(q2)
       s.append(d.toByte.toChar)
       if (r2 != 0 || d > 0x3039) { // check if nano is divisible by 1000000
         s.append((d >> 8).toByte.toChar)
@@ -167,7 +167,7 @@ case class Timestamp private (epochSecond: Long, nano: Int) {
   }
 
   private[this] def append2Digits(x: Int, s: java.lang.StringBuilder): Unit = {
-    val d = digits(x)
+    val d = Timestamp.digits(x)
     val _ = s.append((d & 0xff).toChar).append((d >> 8).toChar)
   }
 
@@ -352,6 +352,11 @@ object Timestamp {
       pos += 2
       ch0 * 10 + ch1 - 528 // 528 == '0' * 11
     }
+    var epochSecond = toEpochDay(
+      year,
+      month,
+      day
+    ) * 86400 + (hour * 3600 + minute * 60 + second)
     var nano = 0
     var ch = (0: Char)
     if (pos < len) {
@@ -372,15 +377,51 @@ object Timestamp {
         }
       }
     }
-    if (ch != 'Z' || pos != len) error()
-    new Timestamp(
-      toEpochDay(
-        year,
-        month,
-        day
-      ) * 86400 + (hour * 3600 + minute * 60 + second),
-      nano
-    )
+    if (ch != 'Z') {
+      val isNeg = ch == '-' || (ch != '+' && {
+        error()
+        true
+      })
+      if (pos + 2 > len) error()
+      var offsetTotal = {
+        val ch0 = s.charAt(pos)
+        val ch1 = s.charAt(pos + 1)
+        if (ch0 < '0' || ch0 > '1' || ch1 < '0' || ch1 > '9') error()
+        pos += 2
+        ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+      } * 3600
+      if (
+        pos + 3 <= len && {
+          ch = s.charAt(pos)
+          pos += 1
+          ch == ':'
+        } && {
+          offsetTotal += {
+            val ch0 = s.charAt(pos)
+            val ch1 = s.charAt(pos + 1)
+            if (ch0 < '0' || ch0 > '5' || ch1 < '0' || ch1 > '9') error()
+            pos += 2
+            ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+          } * 60
+          pos + 3 <= len
+        } && {
+          ch = s.charAt(pos)
+          pos += 1
+          ch == ':'
+        }
+      ) offsetTotal += {
+        val ch0 = s.charAt(pos)
+        val ch1 = s.charAt(pos + 1)
+        if (ch0 < '0' || ch0 > '5' || ch1 < '0' || ch1 > '9') error()
+        pos += 2
+        ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+      }
+      if (offsetTotal > 64800) error() // 64800 == 18 * 60 * 60
+      if (isNeg) offsetTotal = -offsetTotal
+      epochSecond -= offsetTotal
+    }
+    if (pos != len) error()
+    new Timestamp(epochSecond, nano)
   }
 
   private[this] def parseEpochSeconds(s: String): Timestamp = {
diff --git a/modules/core/src-jvm-native/smithy4s/Timestamp.scala b/modules/core/src-jvm-native/smithy4s/Timestamp.scala
index 0fdc6af58..f5be6c212 100644
--- a/modules/core/src-jvm-native/smithy4s/Timestamp.scala
+++ b/modules/core/src-jvm-native/smithy4s/Timestamp.scala
@@ -329,6 +329,11 @@ object Timestamp extends TimestampCompanionPlatform {
       pos += 2
       ch0 * 10 + ch1 - 528 // 528 == '0' * 11
     }
+    var epochSecond = toEpochDay(
+      year,
+      month,
+      day
+    ) * 86400 + (hour * 3600 + minute * 60 + second)
     var nano = 0
     var ch = (0: Char)
     if (pos < len) {
@@ -349,15 +354,51 @@ object Timestamp extends TimestampCompanionPlatform {
         }
       }
     }
-    if (ch != 'Z' || pos != len) error()
-    new Timestamp(
-      toEpochDay(
-        year,
-        month,
-        day
-      ) * 86400 + (hour * 3600 + minute * 60 + second),
-      nano
-    )
+    if (ch != 'Z') {
+      val isNeg = ch == '-' || (ch != '+' && {
+        error()
+        true
+      })
+      if (pos + 2 > len) error()
+      var offsetTotal = {
+        val ch0 = s.charAt(pos)
+        val ch1 = s.charAt(pos + 1)
+        if (ch0 < '0' || ch0 > '1' || ch1 < '0' || ch1 > '9') error()
+        pos += 2
+        ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+      } * 3600
+      if (
+        pos + 3 <= len && {
+          ch = s.charAt(pos)
+          pos += 1
+          ch == ':'
+        } && {
+          offsetTotal += {
+            val ch0 = s.charAt(pos)
+            val ch1 = s.charAt(pos + 1)
+            if (ch0 < '0' || ch0 > '5' || ch1 < '0' || ch1 > '9') error()
+            pos += 2
+            ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+          } * 60
+          pos + 3 <= len
+        } && {
+          ch = s.charAt(pos)
+          pos += 1
+          ch == ':'
+        }
+      ) offsetTotal += {
+        val ch0 = s.charAt(pos)
+        val ch1 = s.charAt(pos + 1)
+        if (ch0 < '0' || ch0 > '5' || ch1 < '0' || ch1 > '9') error()
+        pos += 2
+        ch0 * 10 + ch1 - 528 // 528 == '0' * 11
+      }
+      if (offsetTotal > 64800) error() // 64800 == 18 * 60 * 60
+      if (isNeg) offsetTotal = -offsetTotal
+      epochSecond -= offsetTotal
+    }
+    if (pos != len) error()
+    new Timestamp(epochSecond, nano)
   }
 
   private[this] def parseEpochSeconds(s: String): Timestamp = {
diff --git a/modules/core/test/src-js/smithy4s/TimestampSpec.scala b/modules/core/test/src-js/smithy4s/TimestampSpec.scala
index 64a832e29..170718d3f 100644
--- a/modules/core/test/src-js/smithy4s/TimestampSpec.scala
+++ b/modules/core/test/src-js/smithy4s/TimestampSpec.scala
@@ -78,6 +78,34 @@ class TimestampSpec() extends munit.FunSuite with munit.ScalaCheckSuite {
     }
   }
 
+  property("Converts from DATE_TIME format with timezone offset") {
+    forAll { (i: Date, o: Int) =>
+      val totalOffset = Math.abs(o % 64800)
+      val offsetHours = totalOffset / 3600
+      val offsetMinutes = (totalOffset % 3600) / 60
+      val offsetSeconds = totalOffset % 60
+      val epochSecond = (i.valueOf() / 1000).toLong
+      val nano = (i.valueOf() % 1000).toInt * 1000000
+      val formatted = Timestamp(epochSecond, nano)
+        .format(TimestampFormat.DATE_TIME)
+        .dropRight(1) + {
+        if (offsetSeconds == 0)
+          f"${if (o >= 0) "+" else "-"}$offsetHours%02d:$offsetMinutes%02d"
+        else
+          f"${if (o >= 0) "+" else "-"}$offsetHours%02d:$offsetMinutes%02d:$offsetSeconds%02d"
+      }
+      val ts = Timestamp(
+        epochSecond + {
+          if (o >= 0) -totalOffset
+          else totalOffset
+        },
+        nano
+      )
+      val parsed = Timestamp.parse(formatted, TimestampFormat.DATE_TIME)
+      expect.same(parsed, Some(ts))
+    }
+  }
+
   property("Converts to/from HTTP_DATE format") {
     forAll { (i: Date) =>
       val epochSecond = (i.valueOf() / 1000).toLong
@@ -110,13 +138,9 @@ class TimestampSpec() extends munit.FunSuite with munit.ScalaCheckSuite {
   }
 
   property("Parse EPOCH_SECONDS format with invalid input") {
-    val EpochFormat = """^(\d+)(\.(\d+))?""".r
-    forAll { (str: String) =>
-      val parsed = Timestamp.parse(str, TimestampFormat.EPOCH_SECONDS)
-      parsed match {
-        case Some(_) => expect(EpochFormat.pattern.matcher(str).matches)
-        case None    => expect(!EpochFormat.pattern.matcher(str).matches)
-      }
+    forAll(Gen.alphaStr) { (str: String) =>
+      val parsed = Timestamp.parse(str + "X", TimestampFormat.EPOCH_SECONDS)
+      expect.same(parsed, None)
     }
   }
 
diff --git a/modules/core/test/src-jvm/smithy4s/TimestampSpec.scala b/modules/core/test/src-jvm/smithy4s/TimestampSpec.scala
index 956d0fa1b..e9a1c41c6 100644
--- a/modules/core/test/src-jvm/smithy4s/TimestampSpec.scala
+++ b/modules/core/test/src-jvm/smithy4s/TimestampSpec.scala
@@ -87,6 +87,32 @@ class TimestampSpec() extends munit.FunSuite with munit.ScalaCheckSuite {
     }
   }
 
+  property("Converts from DATE_TIME format with timezone offset") {
+    forAll { (i: Instant, o: Int) =>
+      val totalOffset = Math.abs(o % 64800)
+      val offsetHours = totalOffset / 3600
+      val offsetMinutes = (totalOffset % 3600) / 60
+      val offsetSeconds = totalOffset % 60
+      val formatted = Timestamp(i.getEpochSecond, i.getNano)
+        .format(TimestampFormat.DATE_TIME)
+        .dropRight(1) + {
+        if (offsetSeconds == 0)
+          f"${if (o >= 0) "+" else "-"}$offsetHours%02d:$offsetMinutes%02d"
+        else
+          f"${if (o >= 0) "+" else "-"}$offsetHours%02d:$offsetMinutes%02d:$offsetSeconds%02d"
+      }
+      val ts = Timestamp(
+        i.getEpochSecond + {
+          if (o >= 0) -totalOffset
+          else totalOffset
+        },
+        i.getNano
+      )
+      val parsed = Timestamp.parse(formatted, TimestampFormat.DATE_TIME)
+      expect.same(parsed, Some(ts))
+    }
+  }
+
   property("Converts to/from HTTP_DATE format") {
     forAll { (i: Instant) =>
       val ts = Timestamp(i.getEpochSecond, i.getNano)
@@ -122,8 +148,8 @@ class TimestampSpec() extends munit.FunSuite with munit.ScalaCheckSuite {
 
   property("Parse EPOCH_SECONDS format with invalid input") {
     forAll(Gen.alphaStr) { (str: String) =>
-      val parsed = Timestamp.parse(str, TimestampFormat.EPOCH_SECONDS)
-      expect(parsed.isEmpty)
+      val parsed = Timestamp.parse(str + "X", TimestampFormat.EPOCH_SECONDS)
+      expect.same(parsed, None)
     }
   }