into datetime: noop when input is a datetime (nushell#14845)

# Description

- Closes nushell#14839

When the input to `into datetime` is a datetime, it will return it like
other `into` commands.
# User-Facing Changes

Before, using `into datetime` with a datetime as input would return an
error, now it will return the input.
# Tests + Formatting

Added test `takes_datetime`.
# After Submitting

Doc file is automatically generated.

crates/nu-command/src/conversions/into/datetime.rs

@@ -59,6 +59,7 @@ impl Command for SubCommand {
     fn signature(&self) -> Signature {
         Signature::build("into datetime")
         .input_output_types(vec![
+            (Type::Date, Type::Date),
             (Type::Int, Type::Date),
             (Type::String, Type::Date),
             (Type::List(Box::new(Type::String)), Type::List(Box::new(Type::Date))),
@@ -208,6 +209,12 @@ impl Command for SubCommand {
                 #[allow(clippy::inconsistent_digit_grouping)]
                 result: example_result_1(1614434140_000000000),
             },
+            Example {
+                description: "Leave it as it is when the input is already a datetime",
+                example: "1614434140 * 1_000_000_000 | into datetime | into datetime",
+                #[allow(clippy::inconsistent_digit_grouping)]
+                result: example_result_1(1614434140_000000000),
+            },
             Example {
                 description: "Convert list of timestamps to datetimes",
                 example: r#"["2023-03-30 10:10:07 -05:00", "2023-05-05 13:43:49 -05:00", "2023-06-05 01:37:42 -05:00"] | into datetime"#,
@@ -267,6 +274,11 @@ fn action(input: &Value, args: &Arguments, head: Span) -> Value {
     let timezone = &args.zone_options;
     let dateformat = &args.format_options;
 
+    // noop if the input is already a datetime
+    if matches!(input, Value::Date { .. }) {
+        return input.clone();
+    }
+
     // Let's try dtparse first
     if matches!(input, Value::String { .. }) && dateformat.is_none() {
         let span = input.span();
@@ -636,6 +648,26 @@ mod tests {
         assert_eq!(actual, expected)
     }
 
+    #[test]
+    fn takes_datetime() {
+        let timezone_option = Some(Spanned {
+            item: Zone::Local,
+            span: Span::test_data(),
+        });
+        let args = Arguments {
+            zone_options: timezone_option,
+            format_options: None,
+            cell_paths: None,
+        };
+        let expected = Value::date(
+            Local.timestamp_opt(1614434140, 0).unwrap().into(),
+            Span::test_data(),
+        );
+        let actual = action(&expected, &args, Span::test_data());
+
+        assert_eq!(actual, expected)
+    }
+
     #[test]
     fn takes_timestamp_without_timezone() {
         let date_str = Value::test_string("1614434140000000000");