Calamari supports an overtime processing. Claiming back the overtime hours can be handled this way:
Set up an absence type
Create an Overtime absence type in Configuration->Absence types->Add->Overtime and set up the "Duration unit" in hours.
In Accrual rules set accrue periods as "One-time accrual at the first day of work" and "0" hours.
Adjust the entitlement
Go to Configuration->Employees->choose an employee, click the Adjust entitlement button and put the number of hours that you want to adjust. Also, you can put a minus (-) before to deduct the entitlement.
Then, an employee can request the hours that he worked overtime.