Question: Where Do Time Zones Come From? In email headers after the time, I sometimes see a time zone indication like EDT or CET. I am looking for a complete list of these codes, but couldn’t find it on the Web. – Andree Hollander <[email protected]>
Answer: There are 24 time zones around the world, as you might expect, given the length of the day. The U.S. Naval Observatory has a map of these divisions and the exceptions to them; some states and provinces use half-hour offsets to avoid being further out of sync with the rest of their country or region.
Most mail programs include not only local time, but an offset from what used to be known as Greenwich Mean Time (GMT) but is now known as Universal Time (UT) or a similar variant. GMT is the abbreviation used in mail headers, but the offset you see – like +0700 – is the number of hours to add to local time to equal GMT.
It was peculiarly hard to find a complete list of time zones by country and designation. In the above question, EDT is Eastern Daylight Time and CET is Central European Time. Those are easy enough. But when you get outside of the U.S. and Europe, time zone names become a little… weird. After a long search, I found a list of time zones in a Unix tar archive that had been "gzipped" (compressed using GNU ZIP software), and we’ve put that on the NetBITS site. There’s also an excellent site that explains how the International Standards Organization (ISO) thinks time should be displayed.
On a separate but related note, you’ll notice that if you use a big service provider, like AT&T WorldNet, or a commercial online service, like America Online, the timestamp of your outgoing mail is that of the time zone in which the mail servers physically reside – not necessarily the time zone of your machine. So if you send mail to yourself on the West coast of the U.S. via AOL, it looks like you sent it three hours after you’ve received it.
That reminds me of an anonymous limerick often cited by Isaac Asimov:
There once was a lady named Bright,
Who travelled much faster than light.
She started one day
In a relative way,
And returned on the previous night. [GF]