Harley Hahn's
Internet Advisor


Appendix B...

Time Zones on the Internet

The Internet is used around the world, and times must be expressed carefully, especially within the headers of mail messages and Usenet articles. In general, the Internet uses a 24-hour clock. For example, within a header, you will see 20:50 instead of 8:50 PM. (If you are not used to a 24-hour clock, use the conversion information in Figure B-1 below.)

Aside from the actual time, it is also important to know the time zone. This information is usually expressed in one of three ways.

First, you might see a specific time along with the local time zone. For example, the following header line specifies a time of 8:50 PM, Pacific Daylight Time (PDT):

Date: Sun, 21 Dec 2003 20:50:17 PDT

Another way you might see this same information is with the time converted to GMT (Greenwich Mean Time), also referred to as UT (Universal Time) or UTC (Coordinated Universal Time). You will see GMT/UT/UTC times even when a mail message or Usenet article did not originate in that time zone. The conversion is done automatically by the software. Here is the same time as above specified as GMT:

Date: Mon, 22 Dec 2003 03:50:17 GMT

Notice how, in this example, the GMT time is 3:50 AM one day later. This is because GMT is 7 hours ahead of PDT. For reference, Figure B-2 below summarizes the time zones used in the U.S. and how they compare to GMT.

One last way in which you may see time specified within a header is a local time followed by the number of hours difference from GMT:

Date: Sun, 21 Dec 2002 20:50:17 -0700

Learn how to...

Set the Time and Date on Your Computer

It is up to you to make sure the time and date settings are correct on your computer.

This header line shows the same time, 8:50 PM, and indicates that the local time zone is -7 hours different from GMT.

Whenever your programs need to know the time, date, or time zone, they get the information from settings that are maintained by Windows. For example, when you send a mail message, your mail program puts the date, time, and time zone on the message.

To ensure that your time and date are always correct, you can use a program that will synchronize your computer's clock with an exact time source on the Internet. Once you install one of these programs, it will run on its own in the background, checking the time and date automatically at regular intervals, and making corrections as necessary. However, you do need to make sure your time zone is set correctly.

What's in a Name?

GMT
UT
UTC


Greenwich (pronounced "Gren-itch"), a borough of London, was the home of the Royal Observatory from 1675-1985. It was at this observatory that our modern system of timekeeping and longitude was developed. For this reason, the imaginary north- south line that runs through the observatory is designated as 0 degrees longitude.

In 1884, the time at Greenwich was adopted as the global standard used to determine all the time zones around the world. This global standard time is called Greenwich Mean Time or GMT. (In this context, the word "mean" refers to "average".) GMT is used widely on the Internet, and is sometimes referred to by the newer, more official name of UT (Universal Time).

In addition to UT, you may see another name, UTC (Coordinated Universal Time). UTC is the official value of Universal Time as calculated by the U.S. National Bureau of Standards and the U.S. Naval Observatory.

By the way, you might wonder why the abbreviation for Coordinated Universal Time is UTC, not CUT. Here is why:

UTC was adopted as an official international standard in 1970. The work was done by a group of experts within the International Telecommunication Union. When it came time to name the new standard, the group had a problem.

In English, the abbreviation for Coordinated Universal Time would be CUT. But in French, the name is Temps Universel Coordonné and the abbreviation would be TUC. The group wanted the same abbreviation to be used everywhere, but they couldn't agree on whether it should be CUT or TUC. The compromise was to use UTC. Although the abbreviation is inexact in both English and French, it had the enormous advantage of keeping the peace.

(Not too many people know this, but now you do.)

Figure B-1: Comparison: 24-hour system & AM/PM system

On the Internet, time is often specified using a 24-hour clock. The following table shows the 24- hour values and their equivalent AM/PM times.

12 midnight = 00:00    12 noon = 12:00
    1:00 AM = 01:00    1:00 PM = 13:00
    2:00 AM = 02:00    2:00 PM = 14:00
    3:00 AM = 03:00    3:00 PM = 15:00
    4:00 AM = 04:00    4:00 PM = 16:00
    5:00 AM = 05:00    5:00 PM = 17:00
    6:00 AM = 06:00    6:00 PM = 18:00
    7:00 AM = 07:00    7:00 PM = 19:00
    8:00 AM = 08:00    8:00 PM = 20:00
    9:00 AM = 09:00    9:00 PM = 21:00
   10:00 AM = 10:00   10:00 PM = 22:00
   11:00 AM = 11:00   11:00 PM = 23:00

Figure B-2: U.S. time zones in relation to GMT

Abbreviation Time Zone Difference from GMT
GMTGreenwich Mean Time0
UTUniversal Timesame as GMT
UTCCoordinated Universal Timesame as GMT
ESTEastern Standard Time-5 hours
EDTEastern Daylight Time-4 hours
CSTCentral Standard Time-6 hours
CDTCentral Daylight Time-5 hours
MSTMountain Standard Time-7 hours
MDTMountain Daylight Time-6 hours
PSTPacific Standard Time-8 hours
PDTPacific Daylight Time-7 hours