Hi guys , having a mind block on a time calculation

what i need is to set a time and show the amount of time that has occurred past (OVERTIME ) that set time

the hours mins , sec is working ok as a count up clock

the calculation fails when overtime goes to 1min is sec 0 etc

eg if the set value is say 0 hours , 0 mins 10sec

the overtime clock will start counting past 10sec



Code:
        OT_Hours   = Hours   - set_hour
        OT_Minutes = Minutes - set_Min 
        OT_Seconds = Seconds -set_sec