How do you report hours that vary week to week? after 18 years as an accounts payable clerk

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I worked as an accounts payable clerk before this. The form asks for gross earnings, which I do not see until later. Do you report when you earned it or when you were paid?

Report honestly and keep your stubs. An honest small error is much easier to fix than a missing report.