Time Zones
A time zone is a region on Earth that has a uniform, legally mandated standard time.
Before 1972, all time zones were specified as an offset from Greenwich Mean Time (GMT), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London, United Kingdom. Since 1972, all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT, now called UT1. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, including the United Kingdom itself. UTC, also called Zulu time, is used everywhere on Earth by astronomers and others who need to state the time of an event unambiguously.
Standard Time Zones
Standard time zones can be defined by geometrically subdividing the Earth's spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones would differ by one hour. However, political boundaries, geographical practicalities, and convenience of inhabitants can result in irregularly shaped zones. Moreover, in a few regions, half-hour or quarter-hour differences are in effect.
UTC
If the time is in UTC, add a "Z" directly after the time without a space. "Z" is the zone designator for the zero UTC offset. "09:30 UTC" is therefore represented as "09:30Z" or "0930Z". "14:45:15 UTC" would be "14:45:15Z" or "144515Z".
Time zone as offsets from UTC
Time zone are written as offset from UTC in the format ±[hh]:[mm], ±[hh][mm], or ±[hh]. So if the time being described is one hour ahead of UTC (such as the time in Berlin during the winter), the zone designator would be "+01:00", "+0100", or simply "+01". This is appended to the time in the same way that 'Z' was above. The offset from UTC changes with daylight saving time, e.g. a time offset in Chicago, which is in the North American Central Time Zone, would be "?06:00" for the winter (Central Standard Time) and "?05:00" for the summer (Central Daylight Time).
Time zone as offsets from GMT
Time zones can also be written as offsets from GMT in the format GMT±[h]:[m]
This specifies the number of hours and minutes ahead or behind the GMT reference at Greenwich.
A time zone of GMT would be GMT+0:0, which is normally written as simply GMT.
A time zone that is 5 hours behind the reference GMT would be specified as GMT-5.
If Daylight Savings Time (DST) is also in effect, then your local time goes ahead one hour for the duration of the DST and would become GMT-4.
A time zone that is 5 hours ahead of the reference GMT would be specified as GMT+5.
Again, for DST, it would become GMT+6
If the local time in Greenwich is 3:00 pm on Monday, and your time zone is GMT+5, then your local standard time would be 8:00 pm on Monday.
If your time zone was GMT-5, then your local standard time would be 10:00 am on Monday.
If your local time was 10:00 pm on Monday and your time zone is GMT-5, then the Greenwich local standard time would be 3:00 am on Tuesday.
If your local time is 3:00 am on Tuesday and your time zone is GMT+5, then the Greenwich local standard time would be 10:00 pm on Monday.
You can also reference your local time to other local times.
Let us say you are planning a trip overseas.
You know that your time zone is GMT-5, start zone, and that the country you are visiting has a time zone of GMT+2, end zone.
You can determine the time difference as follows:
difference in hours = (GMT end zone) - (GMT start zone) = (2) - (-5) = 7 hours
This states that the country you are visiting is 7 hours ahead of your local time, so that if your local time is 1:00 pm then the time in your visiting city is 8:00 pm.
If the visiting city had a time zone of GMT-10 then:
difference in hours = (GMT end zone) - (GMT start zone) = (-10) - (-5) = -5 hours
This states that the country you are visiting is 5 hours behind your local time, so that if your local time is 1:00 pm then the time in your visiting city is 8:00 am.
By knowing then, the time difference between your local time and the destination time, along with the number of hours your flight will take, you can determine what the local time would be when you arrive at your destination.
Below is shown various time zones with respect to GMT.
So, if a time zone is 4½ hrs behind GMT, then it would be GMT-4:30.
Somethimes this will be written as GMT-4.5.
The decimal portion of the hour is used instead of the actual minutes of the hour as shown below:
:15 ? .25
:30 ? .5
:45 ? .75
Similarily, if a time zone is 5¾ hrs ahead of GMT, then it would be GMT+5:45.
This can also be written as GMT+5.75
You can also calculate the actual time at your destination.
Consider the following:
You are zone A
Your time zone is GMT-5 so your offset is -5
Your local time is 1:00 pm on Monday
Destination is zone B
Your destination time zone is GMT+2 so the offset here is +2
Conversion between time zones obeys the following:
(time in zone A) ? (GMT offset for zone A) = (time in zone B) ? (GMT offset for zone B)
To solve for our destination time:
(time in zone B) = (time in zone A) ? (GMT offset for zone A) + (GMT offset for zone B)
= (1:00) - (-5) + (2)
= (13:00) - (-5) + (2) : Convert to 24 hour clock
= 13:00 + 5 + 2
= 20:00
= 8:00 pm on Monday
Let us assume you local time was 8:00 pm on Monday, so now we have:
(time in zone B) = (time in zone A) ? (GMT offset for zone A) + (GMT offset for zone B)
= (8:00) - (-5) + (2)
= (20:00) - (-5) + (2) : Convert to 24 hour clock
= 20:00 + 5 + 2
= 27:00
= 24:00 + 03:00 : indicates you have crossed into the next day
= 03:00
= 3:00 am on Tuesday : change back to 12 hour clock
Consider the following:
You are zone A
Your time zone is GMT+5 so your offset is +5
Your local time is 3:00 am on Monday
Destination is zone B
Your destination time zone is GMT-3 so the offset here is -3
time in zone B) = (time in zone A) ? (GMT offset for zone A) + (GMT offset for zone B)
= (3:00) - (+5) + (-3)
= (03:00) - (+5) + (-3) : Convert to 24 hour clock
= 03:00 - 5 - 3
= -05:00
= -24:00 + 19:00 : indicates you have crossed into the previous day
= 19:00
= 7:00 pm on Sunday : change back to 12 hr clock
To convert 24 hr to 12 hr clock, just subtract 12 hr. if the 24 hr time is equal to or greater than 13:00
To convert 12 hr to 24 hr clock, just add 12 hr, if the 12 hr time is equal to or greater that 1:00 pm
00:00 = 12:00 am
06:00 = 6:00 am
12:00 = 12:00pm
13:00 = (13-12) = 1:00 pm
18:00 = (18-12) = 6:00 pm
23:00 = (23-12) = 11:00 pm
24:00 = (24-12) = 12:00 am
(BACK)
United States
