Gets the number of whole years between two dates.
Gets the number of whole years between two date-times, assuming they're in the same time zone.
Gets the number of whole years between two date-times, adjusting the offset of endExclusive if necessary to match the starting date-time.
Gets the number of whole years between two zoned date-times, adjusting the time zone of endExclusive if necessary to match the starting date-time.