How do I input a minus time in an excel spreadsheet

[Nadine NZ]

I am devising a spreadsheet to be used for a timesheet that must calculate
flexi hours. This means that occasionally a minus time will need to be
entered. But when I try to do this excel thinks it's a formula, if I put a '
in front of it, the spreadsheet doesn't work as it is a text value. Does
anyone know a way I can do this ?

 

[Lost for time]

If you go to MSN training, they give good classes on time applications.