I don't really agree with these changes to the documentation.

The format that paulc identifies is actually an RFC 3339 datetime, which is a subset of ISO 8601, to the extent that you consider the fact that "we're using RFC 3339" is "mutual consent" for omitting the `T` separator. The changes to the documentation make it more confusing because it's not clear how precisely the datetime is considered to deviate from the spec.

I think I would change the documentation to refer to the format as RFC 3339 and if ISO 8601 is mentioned at all, it can be parenthetical in the first mention.

In any case, exposing the spec like this would probably be useful:

YYYY-MM-DD HH:MM:SS,mmm

The fact that this complies with the RFC 3339 spec (particularly as commonly implemented) just means "an ISO parser that implements the spec should be able to read this".