Right now the clock just struck 1PM in England. If I went exactly 1000 hours back in time, I don't know exactly what time it would be in England, but it would be the top of the hour regardless. But what if I went 10,000 hours back in time, or 100,000 hours, or a million hours?
My question is, when did our current 24-hour day cycle begin? That is to say, what is the largest number x such that if I started at the top of the hour in England and then traveled exactly x hours back in time, it would still be the top of the hour in England, give or take (say) 5 minutes?
Note that I don't want to know when it was first proposed to divide the day into 24 hours, or even when the duration of the hour first became around 3600 seconds, but rather when the 24-hour day cycle was first at least roughly in sync with how it is right now.