Address lc_time_fmt shortcomings:
- use clock scan to enforce supported date formats, 'YYYY-MM-DD' (undocumented, but explicitly checked for in the code) and 'YYYY-MM-DD HH:MM:SS'
- leverage clock scan capabilities to correctly handle 'impossible dates' such as 1999-02-29
- leverage clock format to obtain the correct week day
- leverage clock api to correctly handle gregorian/julian dates
- implement documented formats that had an empty entry in format array (%W, u, %U, %j)
