Day of a month. Must be from 1 to 31 and valid for the year and month, or 0 to specify a year by itself or a year and month where the day isn't significant.
Month of a year. Must be from 1 to 12, or 0 to specify a year without a month and day.
Year of the date. Must be from 1 to 9999, or 0 to specify a date without a year.