True if the units are time units, False otherwise.
Note that reference time units (such as 'days since 2000-12-1') are not time units.
See also
Examples
>>> print Units('days').istime
True
>>> print Units('kg').istime
False
>>> print Units().istime
False
>>> print Units('hours since 2100-1-1', calendar='noleap').istime
False
>>> print Units(calendar='360_day').istime
False