Brady and Sarah have been messaging on a site for a while now. The site tells you when you sent your messages; but not in exact detail.

For example, Sarah messaged Brady "What's up?" 2 hours and 31 min ago, however it only says "2 hours ago". But when Brady messaged Sarah "Hi!" 47 min ago it said "47 min ago".

(Once it passes the 1 hour mark it is then generalized to how many hours ago)

Brady sent Sarah an important message almost a day ago, he needs to know when, to the exact minute, he sent this. He needs this information very soon. But the site says "23 hours ago".

It's 4:16 PM right now, but for the next 2 days (beginning immediately), he'll have no source of time.

How can Brady find the exact time he sent this message, with the least amount of new messages?

  • 1
    $\begingroup$ Couldn't he just hover the time and get the exact reading in UTC? ::sigh:: I suppose the site could learn something from SE. $\endgroup$ Jul 12, 2015 at 13:53
  • $\begingroup$ @Engineer Toast Haha, I guess this site should improve it's messaging system lol. $\endgroup$
    – warspyking
    Jul 12, 2015 at 14:14
  • $\begingroup$ @EngineerToast Maybe he's on a mobile device and can't hover? $\endgroup$ Aug 28, 2017 at 14:22

1 Answer 1



He just needs 1 new message:
He posts a message immediately. He therefore knows that it was sent at 4:16 PM. The he waits that the time stamp of the past message jumps from "23 hours ago" to "1 day ago". At that point in time he knows that he posted the message exactly 24 h ago. He then looks at his newly posted message, which was posted less than one hour ago, therefore has minutes. He adds those minutes to 4:16 PM, and thus gets the current time. The other post was then posted the previous day at the same time.

  • $\begingroup$ Good job! A quick but fun puzzle, don't you think? $\endgroup$
    – warspyking
    Jul 11, 2015 at 22:36

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