If you calculate with time values, you will soon discover that Excel is not fond of negative time values.
You can bypass this limitation by using the option 1904 date system. You find this setting by using the menu item Tools,
Options…, Calculation. Select this option before you enter any dates or time values in your workbook,
if not you’ll have to edit all the existing date- and time values.
If you want to enter a negative time value you have to do it like this: -”10:15″
Or like this: = -10.25/24
If you prefer not to use the option 1904 date system in your workbook, you can use the IF-function
to separate the positive and negative time values. This gives you the opportunity to have one column
for the positive time values and another column for the negative time values.