Changed date format and validation behavior

- I think there was some problem with date format specifier where "mm" (minute in hour) was used instead of "MM" (month of year), and "hh" (hour in am/pm (1-12)) was used instead of "HH" (hour in day (0-23)).
- It had a timezone specifier which allowed RFC 822 time zone like "-0500". I changed this so that time string always ends with character "Z", meaning that it's UTC. This has an impact on the Aikuma google drive backup process.
- When I wanted to update only one field (date_processed in particular), the validation methods threw an exception because my metadata didn't have other date fields (date_approved and date_backedup). I thought this was a bug.