Cleaned up a lot of leftovers from the Date overhaul. I'm also trending towards using Int64 for as many function arguments as possible to cut down on period conversions (in general, they add unneccesary conversion steps). Also fixed the typemin/typemax for the Date types.
Fixed a timezone bug that wasn't rendering non-UTC timezones correctly before or after the Olsen database dates (< 1901, or > 2038)
Added typemin
and typemax for DateTime type. Also implemented isleap() for DateTimes