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.

  • I'm not sure I understand the question. The 24-hour clock has been used since we decided there were 60 seconds in a minute, 60 minutes in an hour, etc. It's not like back in time it took less or more than 24 hours for the earth to make one rotation... The question should be when the 24-hour clock was first used. – e3ra Oct 30 '17 at 13:29
  • What has your preliminary research shown? Probably the answer involves the last leap year, or perhaps the adoption of BST. What research have you done on time zones and the measurement of time? – Mark C. Wallace Oct 30 '17 at 13:30
  • @ezra I'm not asking when the 24-hour clock was first used, but rather when the 24-hour clock used in such a way that it was at least roughly in sync with our 24-hour clock. If I started at the top of the hour in England and went back 300 years back in time, going exactly a whole number of hours back in time, would it still be the top of the hour in England, or would it be (say) 37 minutes past the top of the hour? – Keshav Srinivasan Oct 30 '17 at 13:35
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    Its really tough to grok what you want out of this question, but my best guess is never. The earth's rotation isn't as regular as we'd all like to imagine, and we often have to add leap seconds to keep our clocks aligned with the earth. Its never been more than one a year that I can tell (except in 1972 when there were two), but some years we don't need them. – T.E.D. Oct 30 '17 at 13:39
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    This is the first question in my memory where I've VtC as "Unclear what you're asking" really truly fits. As you can see, we're all just a bit confused as to what you are really asking. – CGCampbell Oct 30 '17 at 14:32

Since the base unit of time is now defined as the second (minutes, hours and days, etc are all derived from it). The "current 24-hour day cycle" would have been defined as recently as 1997 when the S.I. definition of the second was last changed..

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