If you do something so regularly that all the times you've done it blend into each other, how do you remember if you accidentally miss out doing it one time?

The reason I ask is that my sister and I take it in turns to feed our hamster, and we've done it so many times that (for me, at least) I can't always tell if I'm remembering feeding him yesterday, last week, last month... To help, I've marked the days I fed him on a calendar, but my sister doesn't check it. There have been times when we've disagreed on who fed him last, and my sister rarely believes me if I say it was her, since it's well known in my family that my memory is terrible. No matter how sure I am, it makes no difference, because I just can't prove it and I don't trust my memory enough.

Is there any way I can be absolutely sure who fed him last, or is the calendar the best I can do?

( I realise this is nothing compared to what some people here go through, but it's very frustrating not being believed and knowing that maybe my sister's right not to believe me. And it makes me worry what else I might have forgotten, too. )