If the start and end DateTime values in a time period are not ordered chronologically, they are manipulated to make them so prior to making the comparison, using the rule that end_DateTime >= start_DateTime for all cases.
If a time period contains a null start_DateTime and a non-null end_DateTime, then the values are switched to indicate a non-null start_DateTime and a null end_DateTime.
Implicit casting to a Period data type is not supported.