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This is a simple question, but one I've never seen a definitive answer for:

In the majority of the world that has postal codes, is it safe to assume that given a postal code, there is only one time zone associated with that postal code?

Or alternately, even if it it's not safe in principle, has there ever been an actual case where one postal code spanned two or more time zones?

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    this question is off topic here... but I am curious to know the answer Oct 5, 2017 at 18:48
  • Happy to move it, or have it moved, but I couldn't decide where to put the question. Stack Overflow seemed equally off-topic.
    – user802500
    Oct 5, 2017 at 18:52
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    In countries with daylight saving time, one postal code area may have different timezones at different points of time. I don't have a concrete example, but some postal codes are not geographic but have special uses, e.g. denote an organizational entity. That entity may be present in multiple time zones. You'll probably be able to find an example for military mail postcodes.
    – amon
    Oct 5, 2017 at 19:10
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    Have to vote to close this as off-topic, but I will say that if you have the polygons for time zones and postal zones, that question becomes very easy to answer. Also note that some postal zones don't equate to physical locations.
    – Blrfl
    Oct 5, 2017 at 19:19
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    I think this is a great question - is it really off-topic here? This essentially questions a fairly typical assumption in software engineering. Would it be a better fit if it were framed a little bit differently, e.g. "is it future-proof to store postcodes and timezones in the same DBMS table?". Though I've noticed that similar questions have been decided to be off-topic before and I certainly get your point.
    – Bengt
    Oct 5, 2017 at 21:03

2 Answers 2

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I believe the answer is no although I think it is more about usage. There can be multiple time zones in a single zip. As an example look at the rules for military mail on usps (https://pe.usps.com/text/pub28/28c2_010.htm). There are a few specific zips that include many different time zones around the world.

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It depends on how essential it is you get an accurate timezone, and whether the user can configure their timezone. There are indeed various cities in the world that have multiple timezones. Its safe to assume that there are also postal codes that use different timezones.

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  • Cities divided by two time zones: quora.com/Are-there-any-major-cities-divided-by-two-time-zones
    – user802500
    Oct 5, 2017 at 18:56
  • Sources on cities divided into multiple time zones are light on the ground, and it doesn't necessarily stand to reason that they wouldn't have separate postal codes.
    – user802500
    Oct 5, 2017 at 19:10
  • However, cities are often divided into many separate postal codes. New York City has over 100, for example. The fact that some cities are split into 2 timezones doesn't necessarily mean postal codes would be. Mar 19, 2021 at 19:00

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