Simplify the API by assuming all date element values are handled in DATETIME...

Simplify the API by assuming all date element values are handled in DATETIME format. The CCK Date module handles conversion of ISO and UNIX values to and from DATETIME values. This simplification of the core API should make it easier to get into core and makes the element handling easier to follow, plus I assume the first thing we'll do when a date API is in core is convert core timestamp fields to true DATETIME fields, so this would be the natural default.