I am trying to make a spreadsheet to calculate hours worked etc

The user inputs start and finish times during the week and these are
totalled up.

Then I want to subtract contracted hours from the total and see any extra
hours worked

for example my contracted hours are 27 and I enter that in a cell
I enter in my start and finish times and these are totalled up.

Total hours work - contracted hours = extra
How can I enter a figure for contracted hours which is greater than 24
hours?
What cell format should I use?

many thanks
CH!