I don't know

  • how the master clock, with which other clocks in the world are synchronized, is setup.

  • who decided the mechanism used to setup the master clock

  • when the master clock was setup

  • how to restore the master clock in case all clocks in the world have been scrambled accidentally.

Therefore, let us keep one clock (that has been previously synchronized with the master clock) in a secret place and scramble all other existing ones and destroy all historical data documented based on these clocks.


How to setup a clock that is in sync with the hidden clock's date plus time?


  • There is no trap in this question.

  • You cannot take the hidden clock and use it to synchronize the clock in question. You can make use of the hidden clock only after you have finished setting the clock in question. It means that the hidden clock is available only for comparison.

  • Any historical data (including calendars, diaries, BIOS, etc) created based on the previous clocks are destroyed.

  • The hidden clock is the most accurate clock that is identical to an atomic clock.

  • The scrambled clocks do not show the correct time as the hidden clock shows.

  • $\begingroup$ Uh. What clock is kept? $\endgroup$ – Klyzx Jul 31 '16 at 4:51
  • $\begingroup$ What is wrong with this question that it deserves to be down voted? $\endgroup$ – Money Oriented Programmer Jul 31 '16 at 5:03
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    $\begingroup$ What do you mean by "rebuilding" a clock? I don't think many people on PSE are clock/watch mechanics. $\endgroup$ – mestackoverflow Jul 31 '16 at 5:06
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    $\begingroup$ I didn't downvote it (yet), but I believe the question isn't very clear. For example, can't we just, uh, take out that hidden clock to check it? Also, I don't usually think of clocks as recording dates. If you insist though, I believe people can just remake calendars from memory to keep track of dates if this information was somehow destroyed. Third, how accurate are the clocks in this question? e.g. to the second/millisecond/atomic accuracy? Finally does scrambling the other clocks mean that they definitely do not show the correct time? Just some questions you might want to consider :) $\endgroup$ – yuzuki Jul 31 '16 at 6:06
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    $\begingroup$ Round up all the a**holes who had something to do with the hare-brained scheme of scrambling the master clock, and all the other clocks in the world (except for one).  Find the location of the hidden clock, using torture if necessary. (Heck; torture them even if they talk freely!) Then kill them all and destroy the hidden clock. Finally, set an arbitrarily chosen clock to an arbitrary time, and decree to it be the new master.  Kill anybody who objects. $\endgroup$ – Peregrine Rook Jul 31 '16 at 8:10

Take the clock you are setting up, set it to a date/time that you are sure is in the future, but don't start the clock ticking yet. Then go to the hidden clock and compare it to the time you have set, and keep comparing until the master clock and your clock show the same time. At that point, start your clock going.

You are only setting your clock once (before comparing it to the master), and you are only using the master clock for comparison purposes.

  • $\begingroup$ Ah.. you were sending this while I was typing essentially the same answer... $\endgroup$ – BmyGuest Jul 31 '16 at 8:37

If "Clock A" is the hidden clock (which is the only one in the world that is still set to the correct time), and we want to set "Clock B" to the correct time, but "You cannot take the hidden clock and use it to synchronize the clock in question" (emphasis added; the "clock in question" is "Clock B"), then

  • Grab another clock.  Call it "Clock C".
  • Take "Clock C" to the attic/dungeon where "Clock A" is hidden, and set it to the time showed by "Clock A".
  • Bring "Clock C" out into the daylight, and use it to set "Clock B".
  • $\begingroup$ Thanks for your attempt. See my edit, you cannot make use of the hidden clock except for comparison after you finished setting the clock in question. $\endgroup$ – Money Oriented Programmer Jul 31 '16 at 7:55

Just build any clock and don't set it until the moment of comparison. As there is no knowledge about the "correct" time you can simply bs as long as you want that the clock is correct because nobody can prove you wrong. (If there would be a means to prove you wrong you could use that to set the clock)

Quite likely not the answer as intended but the most likely 'real' solution. Just look at politics for guidance ...


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