| N | |
| Not_computable [Calendar_sig.S.Period] | = Date.Period.Not_computable. | 
| Not_computable [Date_sig.S.Period] | |
| O | |
| Out_of_bounds [Date_sig.S] | 
Raised when a date is outside the Julian period.
 | 
| U | |
| Undefined [Date_sig.S] | 
Raised when a date belongs to
	 [October 5th, 1582; October 14th, 1582]. |