Entering text into a cell that contains a drop down with restricted values

I am creating an electronic timesheet that contains cells in which the time of day is entered. Sometimes, it is necessary to enter text into this cell via a drop down box that includes such options as "Vacation" or "Sick Time". When I now try to type in the dime of day (e.g. 17:00), it gives me an error stating "The value you have entered is not valid. A user has restricted values that can be entered into this cell." How do I allow a cell to have a drop down with a few selectable items while still allowing the times to be manually typed in?

change the error alert to warning instead of stop