Excel - how to show a negative time calculation

[Jim_Free]

I am creating a time sheet that calculates the time used against a starting amount of hours purchased for consulting. I have used the "time" calculation to delete the time used. I have now got to where negative time has been used from the hours purchased. The help section tells me to switch to the "1904 data system" to allow the formula to show a negative number. I have gone to Excel File but do not see the "option" that is explained in help.

Any other suggestions.

For Example: The client has purchased 20 hours of consulting time. 

[#2]

@Jim_Free 

In the desktop version of Excel for Windows:

Select File > Options.

Select Advanced in the navigation pane on the left.

Scroll down to the section 'When calculating this workbook'.

In Excel for Mac, the 1904 date system is the default setting.

[#3]

@Jim_Free 

Another option is to use formula as here

and apply number format to the resulting cell. 

[#4]

Great, thanks! However I'm really afraid of side effects, this is the kind of legacy option which is never tested...
A pity Microsoft does not fix the original issue, the format with brackets [h]:mm:ss;-[h]:mm:ss is supposed to work but this is bugged since forever. :(

[#5]

@Gauthier_BE 

If you don't use the 1904 date system, Excel cannot display negative times. Changing this would probably break other stuff, so it is very unlikely to happen.