Time Zone Abbreviations

What does the abbreviation EST mean?  I'm sure you've seen that before.  If you live in the United States, you might recognize it as "Eastern Standard Time", which is a time zone used in places like New York City, Washington D.C., and Miami.

However, during a significant portion of the year, most places in the United States follow daylight saving time.  During this period, the name for the time zone changes to "Eastern Daylight Time", abbreviated by EDT.  If you continue to use the term EST - you're doing it wrong!

If you want to refer to the entire Eastern Time zone, without regard to whether daylight saving time is in effect or not, the correct abbreviation would be ET.  In other words:

  • EST = Eastern Standard Time = UTC-05:00
  • EDT = Eastern Daylight Time = UTC-04:00
  • ET = Eastern Time = [UTC-05:00/UTC-04:00]

But don't think it's that simple.  The story doesn't end here.

Eastern Time is also used by Canada, and Canada has two official languages - English and French.  While an English-speaking Canadian citizen might say "Eastern Standard Time" or "Eastern Daylight Time", a French-speaking Canadian citizen would say "heure normale de l’Est" or "heure avancée de l’Est" - abbreviated by HNE or HAE.  The other North American time zones have French names too.

Ok, so let's standardize just on the English names.  Then we're ok - right?  Well, not exactly.

You see, America is not the only place with an eastern coastline, and the Australians have also called their eastern time zone "Eastern Standard Time".  If you don't live in Australia, you might be tempted to use the abbreviation AEST, and many have done just that.  But who's to say that the leading A shouldn't stand for America?  Forcing another culture to change their naming conventions to make room for someone else is just plain egocentric.

Additionally, the Australians (like the British) don't use the term "daylight time", they instead call it "summer time", which is appropriate since daylight time always occurs in the summer.  However, that now means EST could refer to "Eastern Summer Time" as well.  So for this reason, in contexts where it's important to distinguish, some times EDT is used anyway, or you may even see AEDT.

To recap, in Australia:

  • [EST/AEST] = [Australian] Eastern Standard Time = UTC+10:00
  • [EST/EDT/AEST/AEDT] = [Australian] Eastern [Summer/Daylight] Time = UTC+11:00
  • [ET/AET] = [Australian] Eastern Time = [UTC+10:00/UTC+11:00]

So now we see that on a global scale, the simple abbreviation EST could refer to any of three different possible time zone offsets.  Therefore, time zone abbreviations are not unique identifiers!  Do not attempt to use them as such.  If you're formatting a string of text to show a user a date and time, sure - you can append a time zone abbreviation.  Humans are usually intelligent enough to apply other contextual clues such as the country and time of year.  But computers do not have such intuition.  You can not parse a string containing a time zone abbreviation and be absolutely certain that it was interpreted correctly.

Need more proof?  Let's look at CST.  Depending on context, it could mean:

  • Central Standard Time (North America) = UTC-06:00
  • Central Standard Time (Australia) = UTC+09:30
  • Central Summer Time (Australia) = UTC+10:30
  • China Standard Time = UTC+08:00
  • Cuba Standard Time = UTC-05:00

How about IST?

  • Indian Standard Time = UTC+05:30
  • Israel Standard Time = UTC+02:00
  • Irish Standard Time = UTC+01:00

That last one is quite interesting, because Irish Standard Time is actually a daylight saving time zone.  Both Ireland and England use the term "Greenwich Mean Time" (GMT) during the winter, but in the summer, England switches to "British Summer Time" (BST) and Ireland switches to "Irish Standard Time" (IST).

It would seem that on a global scale, we cannot agree on the naming conventions for our time zones!

But surely in a single location, one that doesn't follow daylight saving time, we can have a single abbreviation there?  Certainly not!

I'm talking about Hawaii.  Most Hawaiians would say that their time zone is "Hawaii Standard Time" (HST).  But the US legally defines it as "Hawaii-Aleutian Standard Time" (HAST).  So there are two different time zone abbreviations used for Hawaii, that mean the exact same thing.

So why not just use HAST?  Well, "Aleutian" refers to the Aleutian Islands of Alaska, a location 2,423 miles away, and with a population of under 10,000 (as compared with about 1.4 million people living in Hawaii).  Additionally, the Aleutian Islands use daylight saving time in the summer, and Hawaii does not.  Would you change your local customs for a small group of people thousands of miles away?  Probably not.

To recap:

  • HST = Hawaii Standard Time (used by Hawaii only) = UTC-10:00
  • HAST = Hawaii Aleutian Standard Time (used by both) = UTC-10:00
  • HADT = Hawaii Aleutian Daylight Time (used by Aleutian Islands only) = UTC-09:00

Since HADT only exists in the Aleutian Islands, one can only conclude that the good people of the Aleutian islands would rather spend their summers in Hawaii!

Further reading: