Date: consider supporting (or restoring support for?) both "2006-01-02" and RFC3339

[mitranim]

The Date type claims being able to parse RFC3339, but actually uses a short format string. In Go 1.16.2, this fails with parse errors when the input string is RFC3339. I don't know if that's a later breaking change, or was always this way. Either way, it might be useful to actually support both.