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Given the discontinuity in timezones, is there a standard method of specifying which of two repeated clock times you are interested in? This came to my attention from a Java discussion "Why is subtracting these two times (in 1927) giving a strange result?", that was recently brought up, but this is not isolated to strange events in China in the 1920's.

How do I tell you which 01:50 AM (EST) I am interested in on November 3rd this year? I know that I could use UTC, epoch time... but is there even a good way to ask a user which local time they are interested in?

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ChrisBob
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  • The only way to disambiguate the two times is by specifying which time zone offset was in use — either explicitly with a number (UTC-05:00 vs UTC-04:00) or implicitly with an abbreviation (EST vs EDT), or with words like 'the first 01:50 am' or 'the second 01:50 am'. There really isn't any other way: the time 01:50 without any such qualification is (on that day) ambiguous. – Jonathan Leffler Apr 12 '13 at 19:24
  • Note that as it stands, this isn't really programming question. How you *really* specify this will depend on your development environment, such as programming language, libraries, if you are using a sane operating system or not, etc. – Lennart Regebro Apr 13 '13 at 06:24

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How do I tell you which 01:50 AM (EST) I am interested in on November 3rd this year?

By qualifying it as EST, you are already specifying that you mean "Eastern Standard Time" which is UTC-05:00. That is the second instance of 1:50am. The first instance occurred in "Eastern Daylight Time" (EDT) which is UTC-04:00. Of course, had you just said "Eastern Time", then I wouldn't know which one you meant.

However - keep in mind that "EST" by itself is not unique. I made a guess that you meant Eastern Standard Time in the USA - but you could have meant one of three different zones all called the same thing. See here for a list of time zone abbreviations. Now it happens to be that all "EST" zones are in UTC-05:00, but that is just coincidence. There are several zones that have the same name or abbreviation and have different offsets or different DST start/stop dates.

The "standard" you are looking for is part of ISO8601, which allows for dates and times to include an offset. For example, the two points mentioned would be:

2013-11-03T01:50:00-04:00

2013-11-03T01:50:00-05:00

Keep in mind that the offset uniquely identifies the moment/instant in time - but it it does not uniquely identify a time zone. There are several other time zones that use the -04:00 and -05:00 offsets at different times. This point is often forgotten, because the offset is sometimes referred to as the "zone" erroneously.

Matt Johnson-Pint
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