I've figured out how to get it to populate it with the date, using a VBA macro. Is there a way to execute the macro without making the user hit the hotkeys? In other words, does Excel have Triggers or Events?

My VBA code to populate it with the date is (I'm new to VBA, so there may be a simpler way):
For x = 2 To ActiveSheet.UsedRange.Rows.Count
If Trim(ActiveSheet.Cells(x, 13).Value) = "" And Not Trim(ActiveSheet.Cells(x, 14).Value) = "" Then
ActiveSheet.Cells(x, 13).Value = Date
End If
Next