How do you report hours that vary week to week? after 14 years as a security officer

I work some days through a staffing agency while still certifying. There are 14 people counting on me, so I am trying to be careful. 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.