The calendar for reference time units.
May be any string allowed by the calendar CF property.
If it is unset then the default CF calendar is assumed when required.
See also
| Examples: |
|---|
>>> Units(calendar='365_day').calendar
'365_day'
>>> Units('days since 2001-1-1', calendar='noleap').calendar
'noleap'
>>> Units('days since 2001-1-1').calendar
AttributeError: Units has no attribute 'calendar'