Time offset
Encyclopedia
The UTC offset is the time offset from Coordinated Universal Time
(UTC). It is mostly given as hour or hour and minute. Many time zone
s employ two time offsets, one for standard time
and one for daylight saving time
. Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time
(UT).
Take the UTC−05:00 time zone. Its offset is −5 hours, which means that one must add −5 hours to the UTC time to get the UTC−05:00 time (meaning, one must subtract 5 hours from the UTC time). For example, suppose that right now, the local time in the UTC time zone is 4:00 p.m. (16:00 military time
). What is then the local time right now in the UTC−05:00 time zone? The answer: it is 4:00 p.m. + offset = 4:00 p.m. + (−5 hours) = 4:00 p.m. − 5 hours = 11:00 a.m. (in military time: 16:00 + (−05:00) = 16:00 − 05:00 = 11:00).
At the same moment, the local time in the UTC+05:00 time zone is 4:00 p.m. + offset = 4:00 p.m. + (5 hours) = 9:00 p.m. (in military time: 16:00 + (+05:00)= 21:00).
And if the local UTC+05:00 time is 2:00 p.m. (14:00), then at the same moment, the local UTC time is 2:00 p.m. − offset = 2:00 p.m. − (5 hours) = 9:00 a.m. (14:00 − (+05:00) = 14:00 − 05:00 = 9:00).
The quantity offsetto − offsetfrom is of course the time difference between the time zones UTC+offsetfrom and UTC+offsetto. Therefore, to convert from the UTC+offsetfrom time to the UTC+offsetto time, one adds the time difference to the UTC+offsetfrom.
For example, suppose that the current local time in the UTC−05:00 time zone is 9:00 a.m. (this is the “from” time). What is the current local time in the UTC+05:00 time zone (the “to” time)? The answer: we first figure the time difference: it is offsetto − offsetfrom = +05:00 − (−05:00) = 05:00 + 05:00 = 10:00, or ten hours (again one must be careful about subtracting negative numbers). So the local time in the UTC+05:00 zone is 9:00 a.m. + time difference = 9:00 a.m. + 10 hours = 7:00 p.m. (in military time: 9:00 + 10:00 = 19:00).
One should be careful to get the signs right: in converting from one time zone to another (call them the “from” zone and the “to” zone), the time difference is the “to” offset minus the “from” offset. One should (as in any calculation) check that the final result makes sense. The local times in time zones with larger offsets are ahead of the local times in the time zones with smaller offsets. So, for example, any given moment must be a more advanced hour in the UTC+05:00 zone than in the UTC−05:00 zone. And that is indeed what we found above: when it is 7:00 p.m. in UTC+05:00, it is only 9:00 a.m. in UTC−05:00.
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
(UTC). It is mostly given as hour or hour and minute. Many time zone
Time zone
A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
s employ two time offsets, one for standard time
Standard time
Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. Historically, this helped in the process of weather forecasting and train travel. The concept...
and one for daylight saving time
Daylight saving time
Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...
. Before the introduction of UTC on 1 January 1972 Greenwich Mean Time (also known as Zulu time) was the same as Universal Time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...
(UT).
UTC time to UTC+offset time
To convert from UTC to UTC+offset time zones, one adds the offset to the UTC time to get the UTC+offset = local time. The offset should be taken with the proper sign, so that it can be either a positive or a negative number. An example will illustrate.Take the UTC−05:00 time zone. Its offset is −5 hours, which means that one must add −5 hours to the UTC time to get the UTC−05:00 time (meaning, one must subtract 5 hours from the UTC time). For example, suppose that right now, the local time in the UTC time zone is 4:00 p.m. (16:00 military time
24-hour clock
The 24-hour clock is a convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today...
). What is then the local time right now in the UTC−05:00 time zone? The answer: it is 4:00 p.m. + offset = 4:00 p.m. + (−5 hours) = 4:00 p.m. − 5 hours = 11:00 a.m. (in military time: 16:00 + (−05:00) = 16:00 − 05:00 = 11:00).
At the same moment, the local time in the UTC+05:00 time zone is 4:00 p.m. + offset = 4:00 p.m. + (5 hours) = 9:00 p.m. (in military time: 16:00 + (+05:00)= 21:00).
UTC+offset time to UTC time
In this case one must subtract the offset from the UTC+offset, being careful to realize that subtracting a negative number means adding the absolute value of the same number. For example, if the local UTC−05:00 time is 2:00 p.m. (14:00), then at the same moment, the local UTC time is 2:00 p.m. − offset = 2:00 p.m. − (−5 hours) = 2:00 p.m. + 5 hours = 7:00 p.m. (14:00 − (−05:00) = 14:00 + 05:00 = 19:00).And if the local UTC+05:00 time is 2:00 p.m. (14:00), then at the same moment, the local UTC time is 2:00 p.m. − offset = 2:00 p.m. − (5 hours) = 9:00 a.m. (14:00 − (+05:00) = 14:00 − 05:00 = 9:00).
Converting between two arbitrary time zones
To figure how to convert from the UTC+offsetfrom time to the UTC+offsetto time, we first convert the UTC+offsetfrom time to the UTC time, and then the UTC time to the UTC+offsetto time. If the time is x in UTC+offsetfrom, then the time in UTC, according to the above, is x − offsetfrom. Now that we have the UTC time, to get the UTC+offsetto time, we add offsetto to the UTC time, which results in x + offsetto - offsetfrom .The quantity offsetto − offsetfrom is of course the time difference between the time zones UTC+offsetfrom and UTC+offsetto. Therefore, to convert from the UTC+offsetfrom time to the UTC+offsetto time, one adds the time difference to the UTC+offsetfrom.
For example, suppose that the current local time in the UTC−05:00 time zone is 9:00 a.m. (this is the “from” time). What is the current local time in the UTC+05:00 time zone (the “to” time)? The answer: we first figure the time difference: it is offsetto − offsetfrom = +05:00 − (−05:00) = 05:00 + 05:00 = 10:00, or ten hours (again one must be careful about subtracting negative numbers). So the local time in the UTC+05:00 zone is 9:00 a.m. + time difference = 9:00 a.m. + 10 hours = 7:00 p.m. (in military time: 9:00 + 10:00 = 19:00).
One should be careful to get the signs right: in converting from one time zone to another (call them the “from” zone and the “to” zone), the time difference is the “to” offset minus the “from” offset. One should (as in any calculation) check that the final result makes sense. The local times in time zones with larger offsets are ahead of the local times in the time zones with smaller offsets. So, for example, any given moment must be a more advanced hour in the UTC+05:00 zone than in the UTC−05:00 zone. And that is indeed what we found above: when it is 7:00 p.m. in UTC+05:00, it is only 9:00 a.m. in UTC−05:00.