Represents a PDS date value.
month (None or int)
If None, then day is interperated as the day of the year instead
of as the day of the month, and Date serialized in ‘day-of-year’
The day of the month if month is not None. Otherwise it’s the
day of the year.
None or int. Read-only.
Day of year if month is None. Otherwise, day of the month
of month month. int. Read-only.
Return a PDS serialized string (str) representing the object.
Called by str().