One (and only one) EDTF pattern gets rejected as an invalid date format

[kspurgin]

I got this result in both the I8 sandbox and an I8 instance where the EDTF date field, widget, etc have not been customized beyond the settings available in the UI.

This result was with Permit date intervals = false in the form widget settings.

1985-04-12T23:20:30-04 is rejected as an invalid date.

The following (and all other EDTF patterns, given appropriate form widget settings) are accepted:

1985-04-12T23:20:30
1985-04-12T23:20:30Z
1985-04-12T23:20:30+04:30

That only one of all 64 EDTF patterns is rejected makes me suspicious perhaps this is a bug.

Or, if it was intentional because we did not want to descend into the existential hellscape of timezones, I support that decision, but I'd like to document the exception somewhere.

[kspurgin]

meta-issue: #1748