How do you report hours that vary week to week?

My hours change every week at my new part-time job and I am afraid of reporting wrongly. I do not usually post, so bear with me. Pay posts on a different day from when I work. Do you report when you earned it or when you were paid?

Thanks for writing this out. I went through something close to it 9 months ago, and it settled down once I kept a simple log of every call and form.