doc: DART base date for calendars

[hkershaw-brown]

The time manager mod documentation does not base dates.
"Time intervals are stored and defined in terms of integer number of days and integer seconds." from what?
https://docs.dart.ucar.edu/en/latest/assimilation_code/modules/utilities/time_manager_mod.html

[hkershaw-brown]

Default is NO_CALENDAR

[hkershaw-brown]

! List of available calendar types
public :: THIRTY_DAY_MONTHS, JULIAN, GREGORIAN, NOLEAP, NO_CALENDAR, &
GREGORIAN_MARS, SOLAR_MARS

Base dates from the code:

time = set_time(0, 0)
call get_date(time, year, month, day, hour, minute, second)

Calendar	base year	base month	base day	base hour	base minute	base second
THIRTY_DAY_MONTHS	1	1	1	0	0	0
JULIAN	1	1	1	0	0	0
GREGORIAN	1601	1	1	0	0	0
NOLEAP	1	1	1	0	0	0
NO_CALENDAR	NA	NA	NA	NA	NA	NA 🦇 👨
GREGORIAN_MARS	1	1	1	0	0	0
SOLAR_MARS	1	1	1	0	0	0