You want to create a data validation rule to control data entry. Ungroup the weekly sheets, select the range D12:D40 in the Week 1 sheet, and then create a data validation rule with these specifications: Allow Time greater than cell C12. Enter the input message title: End Time. Enter the input message: Enter the end time. (including the period). . Select the Stop error alert style. . Enter the error alert title: Incorrect End Time. . Enter the error message: The end time must be later than the start time. (including the period)