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An embarrassing problem with elementary mathematics (local time computation)

  1. Nov 2, 2012 #1

    I am living in Germany. Let's say that I want to know the local time in New York, and let's arrange that the local time in Germany is 11:00 o'clock (24-hour clock).

    Germany's time is shifted 1 hour ahead (UTC +1), and New York's time is shifted 5 hours behind (UTC -5). So the difference is in this case -6 hours.

    Here comes the problem: when I subtract 6 from 11, I get 5:00 o'clock as a result. But that's not the case.
    The local time in New York is instead 6:00 o'clock.

    Now, let's do another example with Turkey and London. Turkey's time is shifted 2 hours ahead (UTC +2), while London's time is not shifted (UTC 0).
    Let's assume that the local time in Turkey is currently 12:00 o'clock. If I do the math I get 10:00 o'clock which appears to be correct.

    Through both cases I noticed that every time a zero is involved (UTC 0 put on a number line) you get an overflow of 1 - why is that?

  2. jcsd
  3. Nov 2, 2012 #2


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    Staff: Mentor

    New York is currently on (Eastern) Daylight Savings Time ("summer time") which is UTC-4.

    Most of the rest of the USA also uses Daylight Savings Time, but with varying UTC offsets because of the different time zones.

    Early on Sunday morning (4 November) we will "fall back" to standard time (UTC-5 for New York).
  4. Nov 2, 2012 #3


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    Similarly London (and the rest of the uk) has been in british summer time which is UTC+1 for the last 6 months. It went back to UTC+0 last weekend.
  5. Nov 2, 2012 #4
    I see! My problem was that I simply didn't consider the Daylight Savings Times generally, so I was using the wrong UTC offset of New York (UTC-5 instead of UTC-4).

    Therefore it was not really a mathematical issue as I thought before.

    Thanks jtbell and Ryan_m_b for helping me to getting a bit wiser!
    Last edited by a moderator: Nov 2, 2012
  6. Nov 2, 2012 #5


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    No worries :smile:
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