Another factor is that "take X when you wake up Monday and Y when you go to bed Monday" falls apart if you stay up past midnight (then you're taking the second Monday pills on Tuesday), then if you wake up later at 10 PM Monday and go to bed Tuesday afternoon (pretty soon you're taking both of Monday's pills on Tuesday and need to skip taking a day in your pill tracker), whereas recording the last time you took a pill allows you to space them approximately 8 hours apart regardless of the date/time you last took one.