As someone who's been through that dance twice, it's 5 years from the time (well, day) you press "Submit" if you're applying online, or $RANDOM days of Royal Mail nonsense if you choose to apply by post.

I agree though, the Home Office doesn't have a way of knowing where you were fore sure 5 years ago unless they got someone to go through your "days in and out of the UK" list and vetted/cross-referenced it. And even then it'd likely be incomplete and they'd have to guess.

My surmise is that they look at the level of effort you've put in to filling out that detail, and if the total days in/out isn't particularly a borderline case, then they just wave that bit through.