Returns the number of complete days (that is, not including the timestamp day) since and including the base date, 1 January 0001, in the format dddddd (no leading zeros or whitespace characters).

The base date of 1 January 0001 is determined by extending the current Gregorian calendar backward (365 days each year, with an extra day every year that is divisible by 4 except century years that are not divisible by 400). It does not take into account any errors in the calendar system that created the Gregorian calendar originally.