APDL will not allow the Duty On time to be set after the departure time of the first leg. This includes a canceled leg in the event that the leg is canceled after you report for duty. The Canceled Prior To Duty payroll category is intended to be used when a leg is canceled prior to reporting for duty and your Duty On time is therefore adjusted to a later time. If the Canceled Prior To Duty payroll category is selected, APDL will allow you to adjust the Duty On time after the scheduled departure time of the canceled leg. This allows your duty calculations to be correct while also allowing you to keep the canceled leg in APDL for payroll tracking purposes.
See the Payroll Settings page for more details on payroll settings.