We're testing round-tripping date properties in the dart SDK and noticed that json serialization behaves inconsistently. For example, the date `-0001-01-01 00:00:00` - I.e. year 1 BCE is encoded as `-001-01-01 00:00:00` on Apple platforms, but as empty string on Windows (looks like all dates equal to or below 0000-01-01 00:00:00 get encoded as empty on Windows).

The date 0000-01-01 00:00:00 gets encoded as 0000-01-01 00:00:00 on Apple platforms but 0-01-01 00:00:00 (note the lack of leading zeroes) and as 1905-06-06 18:35:44 on Android.

The date -271821-04-21 00:00:00 gets encoded as 2016-09-19 11:52:32 on Android, but correctly on Apple platforms.

It is possible that there's also a bug in the dart SDK in the way we represent dates on some platforms, but I verified that the reported that it's a serialization bug on windows (Studio correctly visualizes the expected date) and non-negative/zero dates work just fine, so there are strong indications that the bug is in the to_json implementation in Core.