This may help you as well. This is a conditional statement that will fill the cell with a default date if it is empty but will subtract one hour if it is a valid date/time and put it into the cell.
=IF((Sheet1!C4)="",DATE(1999,1,1),Sheet1!C4-TIME(1,0,0))
You can also substitute TIME
with DATE
to add or subtract a date or time.