Your SignInTime really should be a timestamp as well; don't firefighters work overnight? Then a calculation (result is time) would be:
Get ( CurrentTimeStamp ) - SignInTime
In the calculation box, then go to Storage Options (bottom right) and check 'do not store calculation results...' so it will update whenever that layout is viewed. This is important with most Get() functions; they must be unstored to update.
To format the display of time, handle that from your layout itself (Inspector > Data tab and select at the bottom Data formatting (and the clock) and specify displaying hours, minutes as you wish. :^)