Populate a cell with a specific term when the adjacent cell is populated - Overtime form

I'm filling in my overtime form at work, and am learning Excel as I go. One thing I'm trying to achieve is when I enter data into a cell on a particular row (specifying that I did overtime on that date), the start time of that overtime (always 17:00) will automatically be entered into the next cell in the row.

If there is no data in the cell, then the next cell with remain empty.

I am using Excel 2007.

Thanks for any help.