> For what parts of ISO 8601 to accept, there's a standard: RFC3339

This is almost indistinguishable from the idea of accepting .isoformat() and str() results.  From what I see the only difference is that 't' is accepted for date/time separator and 'z' is accepted as a timezone.

Let's start with this.

As an ultimate solution, I would like to see something like codec registry so that we can do things like datetime(.., format='rfc3339') or date(.., format='gnu') for GNU parse_datetime.  I think this will look more pythonic than strptime().  Of course, strptime format can also be accepted as the value for the format keyword.