Replaced some conditional variables with early return statements.
Introcuded Date::Calc's check_date() and check_time() to assure
for integrity of date/time values submitted to according setter
methods.Wrapped check_date() and check_time() in subs which croak upon
error and supply the type of value and the actual value in the
message delivered to the caller, otherwise return true.Separated datetime_parse_duration() logic from datetime_parse()
and added according documentation [Cory Watson jheephat@gmail.com]