Add a couple more supported date formats

[MFAshby]

Feel free to tell me that I'm holding it wrong regarding use of inline-for over decls to select a date format. I considered a ComptimeStringMap -> function pointer as an alternative, but I think this is easier to read and understand.

[kristoff-it]

Thank you, I've created #32 as a followup (assigned to me since it requires some design work).