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