The question is not clear so here are some suggestions

B1: =A1-0.5
A1: 11:00 PM --- B1: 11:00 AM

=IF(A10.5,A1-0.5,A1)

Menu:DataValidation... Allow:Time, Data:Less then or Equal to, End
time: 12:00


Hope it helped
Ola Sandström


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