How do you report hours that vary week to week? - 7 months and counting

I work some days through a staffing agency while still certifying. 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?

Thanks for writing this out. I went through something close to it last year, and it got easier once I kept a simple log of every call and form.

I estimated and then checked my pay stub. When there was a small difference I wrote to the agency to correct it.