Time zones 101: Conversions & Etiquette

[DaveShack]

Fellow citizens,

I've noticed a big increase in the incidence of problems with time zones. This is quite natural, as we have many new people who may not have had the experience of global communications. I've decided to start this thread to serve as a reference for time zones, and for people to ask questions and get help converting time zones.

First some definitions:

GMT / UTC
Greenwich Mean Time, or Universal Time Coordinated, is the reference point from which the time zones are mapped. Roughly speaking, this time zone is centered on Greenwich England (Brits may correct terminology ). Zones to the west of GMT/UTC are earlier, and zones to the east are later. This is true of comparing any two zones.

Commonly used zones
This list is not meant to be chauvinistic in favor of the US and Europe -- these are just the time zones I know offhand. Please feel free to post your time zone's name and GMT +/- correction. You can also find other references to time zone information at
World Clock
World Time Zone
Time Zone Converter
Wikipedia : Time Zones

US
Eastern Time GMT-5 New York, Washington DC, Massachusets, Georgia, Florida, etc.
Central Time GMT-6 Ohio, Illinois, Missouri, Texas, etc.
Mountain Time GMT-7 Colorado, Wyoming, Arizona, New Mexico, Montana, etc.
Pacific Time GMT-8 California, Washington (state), Oregon, etc.

Europe
Central European Time GMT+1 Netherlands, Germany, Sweden, Norway (I'm not really sure about some of these, use references above.)

12 and 24 hour clock
The non-metric countries tend to use a 12 hour clock with AM and PM. AM means "ante meridian" and covers the time between midnight and noon. PM means "post meridian" and covers the time between noon and midnight. Using AM and PM can get very confusing to people who are used to using a 24 hour clock. The 12:00 hours are the ones which cause the most confusion.

Typically metric countries use a 24 hour clock, with 0000=2400=midnight and 1200=noon. This eliminates any possible confusion about morning and night. People who are used to AM and PM have trouble with this system because they have to add or subtract 12 to convert.

Conversion:
12 hour to 24 hour: If PM, add 12 hours, if AM do nothing. Don't specify either AM or PM in the result.
24 hour to 12 hour: If hour is 00, use 12:xx AM. If 1 <= hour <= 11, use the hour and AM. If 12 <= hour <= 23, use hour - 12 and PM. 00:00 is 12:00 AM, 12:00 is 12:00 PM.
Tools: Or just use one of the handy tool references above. Put in a time and location, and a target location, and the tool does the work for you.

Daylight Savings Time / Summer Time
In many parts of the world, but not all, the time is adjusted during summer to place more hours of light during the time that agrarian (farming) people are awake. The rule is "spring forward, fall back". If your location uses Daylight or Summer time, you turn your clock forward one hour on the 1st day that DST is observed, and back one hour on the day after the last day of DST. By definition, GMT / UTC is NOT converted to DST. This means that if your location uses DST, the adjustment factor is changed by one, by adding.

GMT-5 standard time (EST) becomes GMT-4 daylight time (EDT).

Time zone ettiquite
It is a good idea to always post the time in your timezone (so you know when it is), your GMT conversion factor, and the GMT time in 24-hour format. This lets others check your work on the conversion factor, and easily convert to their time zone. You may also choose to post other common time zones already converted, if you're sure you have the conversion right. Always post times for a given zone in the format prevalent in that zone, i.e. 24 hour in Europe, 12 hour in the US.

[Furiey]

Excellent DS.

Daylight Savings time causes a lot of confusion with GMT, as it tends to be assumed that when the UK goes to summer time, GMT changes as well. As you pointed out, GMT does not change with daylight savings. This means that the UK is NOT on GMT in the summer, but GMT+1 (or British Summer Time - BST).

[BCLG100]

Originally Posted by Furiey

Excellent DS.

Daylight Savings time causes a lot of confusion with GMT, as it tends to be assumed that when the UK goes to summer time, GMT changes as well. As you pointed out, GMT does not change with daylight savings. This means that the UK is NOT on GMT in the summer, but GMT+1 (or British Summer Time - BST).

I made that mistake many a time

[ravensfire]

Originally Posted by DaveShack

US
Eastern Time GMT-5 New York, Washington DC, Massachusets, Georgia, Florida, etc.
Central Time GMT-6 Ohio, Illinois, Missouri, Texas, etc.
Mountain Time GMT-7 Colorado, Wyoming, Arizona, New Mexico, Montana, etc.
Pacific Time GMT-8 California, Washington (state), Oregon, etc.

Something to add here, and related to a later remark, these are all Standard times (EST, CST, MST, PST). In a few months, we'll transition to Daylight Savings Time (EDT, CDT, MDT, PDT). When that happens, the GMT conversion number will drop by one (-4, -5, etc).

Yes, it's a pain. Write a program that's in use world-wide and these pains just don't stop!

-- Ravensfire

[Chieftess]

Stickied for future reference.




(Don't forget RMT, either! -- inside joke)