Method 1: Changing Excel's Default Date System
One quick and easy way to display negative values is to simply press the Office button, go to Advanced (pre-2007, Tools → Options → Calculation; on the Mac, Excel → Preferences... → Calculation), and check the "Use 1904 date system" checkbox. Enter 5:00:00 AM in cell A2. In cell A3, enter 6:00:00 AM. In cell A4, type =A2-A3.
You will get the result of -1:00, but only if you checked the 1904 date system checkbox as described.
The 1904 date system is also called Macintosh dates and times.
Nowyou will be able to subtract times from each other and have the result displayed as a negative time value.
Be aware that doing this will cause Excel to change the starting dates from which all cells are calculated from 1 January 1900 to 2 January 1904. Changing this option will affect only the workbook in which you make the change.
If changing Excel's default date system to the 1904 date system is likely to cause problems within the workbook for other time calculations, you need to use another method that will change the appearance of a cell housing a negative value.
