Make `DateTime(::TimeDate)` and `ZonedDateTime(::TimeDateZone)`

[omus]

You could simplify and reduce duplicate code if you made the following functions:

DateTime(::TimeDate)
ZonedDateTime(::TimeDateZone)

For example you could use ZonedDateTime(::TimeDateZone) to remove several lines from your astimezone:

TimesDates.jl/src/timezone/astimezone.jl

Lines 7 to 10 in cdd6105

	fast_time = fasttime(at_time)
	slow_time = at_time - fast_time
	in_zone = x.in_zone
	zdt = ZonedDateTime(on_date+slow_time, in_zone)

[JeffreySarnoff]

thanks

[JeffreySarnoff]

followed