implement Ord and PartialOrd for DateTime (#2653)

## Motivation and Context

This change will allow easy sorting or comparing anything that contains
a DateTime. My example is wanting to sort a list of S3 objects by last
modified.

This PR fixes #2406

## Description
It's a pretty small PR, it implements the `Ord` trait for `DateTime` by
comparing the `seconds` property of `self` and `other`, if they are
equal it compares the `subsec_nanos` properties instead.

The `PartialOrd` trait is implemented by calling the `Ord` trait.

## Testing
I added a unit test that compares a number of `DateTime` values with
different combinations of positive/zero/negative
`seconds`/`subsec_nanos`.

## Checklist
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Author: henriiik

CHANGELOG.next.toml

@@ -17,3 +17,9 @@ author = "rcoh"
 references = ["smithy-rs#2612"]
 meta = { "breaking" = false, "tada" = false, "bug" = false }
 
+
+[[smithy-rs]]
+message = "Implement `Ord` and `PartialOrd` for `DateTime`."
+author = "henriiik"
+references = ["smithy-rs#2653"]
+meta = { "breaking" = false, "tada" = false, "bug" = false }

rust-runtime/aws-smithy-types/src/date_time/mod.rs

@@ -9,6 +9,7 @@ use crate::date_time::format::rfc3339::AllowOffsets;
 use crate::date_time::format::DateTimeParseErrorKind;
 use num_integer::div_mod_floor;
 use num_integer::Integer;
+use std::cmp::Ordering;
 use std::convert::TryFrom;
 use std::error::Error as StdError;
 use std::fmt;
@@ -301,6 +302,21 @@ impl From<SystemTime> for DateTime {
     }
 }
 
+impl PartialOrd for DateTime {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for DateTime {
+    fn cmp(&self, other: &Self) -> Ordering {
+        match self.seconds.cmp(&other.seconds) {
+            Ordering::Equal => self.subsecond_nanos.cmp(&other.subsecond_nanos),
+            ordering => ordering,
+        }
+    }
+}
+
 /// Failure to convert a `DateTime` to or from another type.
 #[derive(Debug)]
 #[non_exhaustive]
@@ -552,4 +568,32 @@ mod test {
             SystemTime::try_from(date_time).unwrap()
         );
     }
+
+    #[test]
+    fn ord() {
+        let first = DateTime::from_secs_and_nanos(-1, 0);
+        let second = DateTime::from_secs_and_nanos(0, 0);
+        let third = DateTime::from_secs_and_nanos(0, 1);
+        let fourth = DateTime::from_secs_and_nanos(1, 0);
+
+        assert!(first == first);
+        assert!(first < second);
+        assert!(first < third);
+        assert!(first < fourth);
+
+        assert!(second > first);
+        assert!(second == second);
+        assert!(second < third);
+        assert!(second < fourth);
+
+        assert!(third > first);
+        assert!(third > second);
+        assert!(third == third);
+        assert!(third < fourth);
+
+        assert!(fourth > first);
+        assert!(fourth > second);
+        assert!(fourth > third);
+        assert!(fourth == fourth);
+    }
 }