Update documentation to clarify what subset of RFC3339 we support for dates

[andrewleader]

Someone pointed out that we aren’t fully supporting RFC3339. Specifically, we don’t support fractional seconds and leap seconds (and possibly more, those were the examples they pointed out)

Fractional seconds 1985-04-12T23:20:50.52Z

Leap seconds 1990-12-31T23:59:60Z

This represents the leap second inserted at the end of 1990.

We should document this, and document what we actually support.

[ghost]

This issue has been automatically marked as stale because it has not had any activity for 5 days.

[RebeccaAnne]

See also #2925

[ghost]

Hi @andrewleader. We have acknowledged this issue report. Please continue to follow this issue for updates/progress/questions.