Because we are an international clan with members from all across the globe this continues to be a recurring problem and it's getting ridiculous having to explain it all the time. Almost all of the confusion stems from everyone not understanding GMT/UTC and Daylight Savings Time (abbreviated DST). So, here is a quick and dirty rundown on time. Firstly;
The 24 Hour Clock
Popularly called "Military Time" this is the international standard of time notation. Rather than segmenting a day into two 12 hour halves, military time simply reads the exact hour and minute of the day. The day begins at midnight (12am) and is noted as 00:00, 8am is 08:00, 4pm is 16:00, etc. Noting time this way leaves no confusion as to whether "8 O'Clock" is 8pm or 8am. Doing time "time zone math" is also much easier in military time. Here's a chart for anybody who might still be confused.
GMT and UTC
They're the same. In all practical purpose 20:00 UTC (Coordinated Universal Time) is the exact same as 20:00 GMT (Greenwich Mean Time), the difference between the two is literally a fraction of a second. UTC can also be written in Zulu time. 16:00z = 16:00GMT = 4pm UTC. UTC replaced GMT way back in 1972, it's based off of atomic clocks, it's hyper accurate and that is very important for computers. As for why most people still say GMT, I don't know. Perhaps it's the same reason people still believe Marconi invented the radio, they've just never gotten around to changing the textbooks. Now's your chance to be hip and keeping up with the times, use UTC.
Writing down the time in UTC is the easiest and most reliable way of scheduling an event with people across many timezones. GMT DOES NOT EQUAL THE TIME IN BRITAIN, GMT/UTC IS CONSTANT YEAR ROUND, DST OR NOT but I'll come back to that one later. For now you need to know what timezone you are in; here is a map of all timezones:
Go ahead and expand it to full size so that you can read it.
Check out the pretty colors; different colors demarcate different timezones. For various reasons (namely practicality) these timezones are arranged politically at a somewhat appropriate longitude. E.g. Most of Spain is at roughly the same longitude as Great Britain, but it is lumped together with most of the continental European countries into Central European Time.
At the bottom center of the map, there is an explanation on how to convert your local time into UTC or vice versa. Find your location on the map, there is a number at the top and bottom of the map over your timezone. This is your standard UTC time difference in hours. If you are to the West of the Prime Meridian (0* longitude) you are -x hours from UTC, if you are to the East, you are +x hours from UTC. So, if you're sipping coffee at 10am Friday morning in Seattle and you're wondering what time it is in UTC, you know that you are in timezone UTC-8, so simply add 8 hours to your 10am local time and you should come to 4pm UTC or 16:00z. If Mr. Seattle wants to know what time it is in Sydney he needs to add another 10 hours difference on top of his 8 ours, meaning it is exactly midnight, 12am, 00:00 on Saturday morning.
It's just simple math, addition and subtraction. once you've got this down firmly you can move on to the part that screws just about everyone...
Daylight Savings Time
To be repeated; GMT DOES NOT EQUAL THE TIME IN BRITAIN. This is the biggest mistake people make. UTC/GMT IS CONSTANT YEAR ROUND, DST OR NOT. This mistake arises from our western bias since most of our clan members are from US/Canada and Europe. These regions' Standard Time Season is the half of the year from early November to mid March (exact date and time of switch varies locally) which correlates with the colder seasons of Fall and Winter. During this Standard Time Season 4pm UTC is indeed 4pm in Great Britain (at UTC+0), 5 pm Central European Time (at UTC +1), and 11am Eastern Standard Time (UTC -5). During this time we all get used to the I'm a +1 or a -5 from UTC attitude and life is good. However, come the Spring switch we get ignorant. Here is a map of countries around the world.
Regions in blue participate in DST, those in orange and red have either abolished DST or have never participated.
Clearly the entire world does not change their clocks, and with the extra complexity DST creates can you blame them? Come Spring, people in USA/Canada and Europe move their clocks ahead one hour, while the few regions in the Southern Hemisphere who participate in DST (half of Australia and Brazil, New Zealand, and Chile) are doing the opposite. For them (Northern) Spring is Fall and they are moving their clocks back an hour to Standard Time. So, since us hooligans up in the Northwest corner of the map are still in sync relative to each other we continue on our merry way with our usual +1 or -5 calculation but this is incorrect.
One more time: GMT DOES NOT EQUAL THE TIME IN BRITAIN. When the Brits do DST they aren't pushing UTC time ahead one hour, they enter British Summer Time which is UTC +1. This happens in every region. For each region pushing their clocks ahead one hour, you transition from your standard timezone to a different DST/Summertime zone. If you're in the Eastern US, you are now in Eastern Daylight Time (UTC-4) as opposed to your standard fall/winter Eastern Standard Time (UTC-5). In continental Europe you're transitioning from Central European Time (UTC +2) to Central European Summer Time (UTC +2).
WHEN YOUR REGION ENTERS DAYLIGHT SAVINGS TIME, ADD ONE HOUR TO YOUR TIMEZONES UTC DIFFERENCE
USA/Canada
Europe
*Russia came to their senses and abolished DST in 2011.
Know your local rules regarding time, I know some places have offset timezones such as Newfoundland or India which is 30 minutes ahead of Pakistan to the West and 30 minutes behind Bangladesh to the East. There are also some oddball places in regards to DST such as Arizona and Saskatchewan.
There are a few funny weeks each year where the US/Canada and Europe are out of whack since we start DST on different dates, but those exact days shift year to year and we'll deal with it when we get there.
The 24 Hour Clock
Popularly called "Military Time" this is the international standard of time notation. Rather than segmenting a day into two 12 hour halves, military time simply reads the exact hour and minute of the day. The day begins at midnight (12am) and is noted as 00:00, 8am is 08:00, 4pm is 16:00, etc. Noting time this way leaves no confusion as to whether "8 O'Clock" is 8pm or 8am. Doing time "time zone math" is also much easier in military time. Here's a chart for anybody who might still be confused.
| 00:00 | 12am (Start of Day) |
| 01:00 | 1am |
| 02:00 | 2am |
| 03:00 | 3am |
| 04:00 | 4am |
| 05:00 | 5am |
| 06:00 | 6am |
| 07:00 | 7am |
| 08:00 | 8am |
| 09:00 | 9am |
| 10:00 | 10am |
| 11:00 | 11am |
| 12:00 | 12pm (Noon) |
| 13:00 | 1pm |
| 14:00 | 2pm |
| 15:00 | 3pm |
| 16:00 | 4pm |
| 17:00 | 5pm |
| 18:00 | 6pm |
| 19:00 | 7pm |
| 20:00 | 8pm |
| 21:00 | 9pm |
| 22:00 | 10pm |
| 23:00 | 11pm |
| 24:00 | 12am (end of Day) |
GMT and UTC
They're the same. In all practical purpose 20:00 UTC (Coordinated Universal Time) is the exact same as 20:00 GMT (Greenwich Mean Time), the difference between the two is literally a fraction of a second. UTC can also be written in Zulu time. 16:00z = 16:00GMT = 4pm UTC. UTC replaced GMT way back in 1972, it's based off of atomic clocks, it's hyper accurate and that is very important for computers. As for why most people still say GMT, I don't know. Perhaps it's the same reason people still believe Marconi invented the radio, they've just never gotten around to changing the textbooks. Now's your chance to be hip and keeping up with the times, use UTC.
Writing down the time in UTC is the easiest and most reliable way of scheduling an event with people across many timezones. GMT DOES NOT EQUAL THE TIME IN BRITAIN, GMT/UTC IS CONSTANT YEAR ROUND, DST OR NOT but I'll come back to that one later. For now you need to know what timezone you are in; here is a map of all timezones:
Go ahead and expand it to full size so that you can read it.
Check out the pretty colors; different colors demarcate different timezones. For various reasons (namely practicality) these timezones are arranged politically at a somewhat appropriate longitude. E.g. Most of Spain is at roughly the same longitude as Great Britain, but it is lumped together with most of the continental European countries into Central European Time.
At the bottom center of the map, there is an explanation on how to convert your local time into UTC or vice versa. Find your location on the map, there is a number at the top and bottom of the map over your timezone. This is your standard UTC time difference in hours. If you are to the West of the Prime Meridian (0* longitude) you are -x hours from UTC, if you are to the East, you are +x hours from UTC. So, if you're sipping coffee at 10am Friday morning in Seattle and you're wondering what time it is in UTC, you know that you are in timezone UTC-8, so simply add 8 hours to your 10am local time and you should come to 4pm UTC or 16:00z. If Mr. Seattle wants to know what time it is in Sydney he needs to add another 10 hours difference on top of his 8 ours, meaning it is exactly midnight, 12am, 00:00 on Saturday morning.
It's just simple math, addition and subtraction. once you've got this down firmly you can move on to the part that screws just about everyone...
Daylight Savings Time
To be repeated; GMT DOES NOT EQUAL THE TIME IN BRITAIN. This is the biggest mistake people make. UTC/GMT IS CONSTANT YEAR ROUND, DST OR NOT. This mistake arises from our western bias since most of our clan members are from US/Canada and Europe. These regions' Standard Time Season is the half of the year from early November to mid March (exact date and time of switch varies locally) which correlates with the colder seasons of Fall and Winter. During this Standard Time Season 4pm UTC is indeed 4pm in Great Britain (at UTC+0), 5 pm Central European Time (at UTC +1), and 11am Eastern Standard Time (UTC -5). During this time we all get used to the I'm a +1 or a -5 from UTC attitude and life is good. However, come the Spring switch we get ignorant. Here is a map of countries around the world.
Regions in blue participate in DST, those in orange and red have either abolished DST or have never participated.
Clearly the entire world does not change their clocks, and with the extra complexity DST creates can you blame them? Come Spring, people in USA/Canada and Europe move their clocks ahead one hour, while the few regions in the Southern Hemisphere who participate in DST (half of Australia and Brazil, New Zealand, and Chile) are doing the opposite. For them (Northern) Spring is Fall and they are moving their clocks back an hour to Standard Time. So, since us hooligans up in the Northwest corner of the map are still in sync relative to each other we continue on our merry way with our usual +1 or -5 calculation but this is incorrect.
One more time: GMT DOES NOT EQUAL THE TIME IN BRITAIN. When the Brits do DST they aren't pushing UTC time ahead one hour, they enter British Summer Time which is UTC +1. This happens in every region. For each region pushing their clocks ahead one hour, you transition from your standard timezone to a different DST/Summertime zone. If you're in the Eastern US, you are now in Eastern Daylight Time (UTC-4) as opposed to your standard fall/winter Eastern Standard Time (UTC-5). In continental Europe you're transitioning from Central European Time (UTC +2) to Central European Summer Time (UTC +2).
WHEN YOUR REGION ENTERS DAYLIGHT SAVINGS TIME, ADD ONE HOUR TO YOUR TIMEZONES UTC DIFFERENCE
USA/Canada
| Time Zone | Standard Time | Daylight Savings Time |
| Atlantic Time Zone | AST (UTC-4) | ADT (UTC-3) |
| Eastern Time Zone | EST (UTC-5) | EDT (UTC-4) |
| Central Time Zone | CST (UTC-6) | CDT (UTC-5) |
| Mountain Time Zone | MST (UTC-7) | MDT (UTC-6) |
| Pacific Time Zone | PST (UTC-8) | PDT (UTC-7) |
| Alaska Time Zone | AKST (UTC-9) | AKDT (UTC-8) |
| Hawaiian-Aleutian Time Zone | HAST (UTC-10) | HADT (UTC-9) |
Europe
| Time Zone | Standard Time | Daylight Savings Time |
| Greenwich Mean Time | GMT (UTC+0) | British Summer Time (UTC+1) |
| Central European | CET (UTC+1) | CEST (UTC+2) |
| Eastern European | EET (UTC+2) | EEST (UTC+3) |
| Russia (Moscow)* | MSK (UTC+4) | MSK (UTC+4) |
Know your local rules regarding time, I know some places have offset timezones such as Newfoundland or India which is 30 minutes ahead of Pakistan to the West and 30 minutes behind Bangladesh to the East. There are also some oddball places in regards to DST such as Arizona and Saskatchewan.
There are a few funny weeks each year where the US/Canada and Europe are out of whack since we start DST on different dates, but those exact days shift year to year and we'll deal with it when we get there.
Last edited by Max on Sat Jun 18, 2011 6:47 am; edited 6 times in total
