If the field has been correctly interpreted as a date or timestamp, the numeric value is used at all numeric operations. Which means that the formatting isn't interesting.
Examples:
If Field1 is a date '12/31/2012' and Field2 is a date '2012-12-31', then Field1=Field2 will evaluate as TRUE.
Fabs(Field1-Field2) < 10/24/60 will return TRUE if the field values are less than 10 min apart.