You can use Data Validation with a list. Follow the steps below.
On one of your worksheets, create a list of allowable time entries. The example creates a list that has time entries spaced 5 minutes apart:
Select the entire list (not including the header!), and give it a name by clicking in the text box to the left of the formula box and entering a name:to the left of the formula box and entering a name:
Then select the cell(s) that you want the drop-down to appear in, and go to the Data tab on the Ribbon. Click on Data Validation, select Data Validation... and then pick "List" from the drop-down. In the Source box, enter "=MyTimeList" (without the quote marks).
Now you should have drop-downs for selecting time!
Hope this helps,
Eric