Since Excel stores dates and times as numeric values, it's possible to add or subtract one from the other.

However, if you have a workbook containing only times (no dates), you may have discovered that subtracting one time from another doesn't always work. Negative time values appear as a series of hash symbols, even though you've assigned the [h]:mm format to the cells. By default, Excel uses a date system that begins with January 1, 1900. A negative time value generates a date/time combination that falls before this date, which is invalid.

Caption: Negative time values are displayed correctly when Excel's 1904 date system is used

The solution is to use the optional 1904 date system. Select ToolsûOptions, click the Calculation tab and check the 1904 date system box to change the starting date to January 2, 1904. Your negative times will now be displayed correctly.