# UTC offset

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UTC offset

The UTC offset is the time offset from Coordinated Universal Time (UTC). It is mostly given as hour or hour and minute. Many time zones 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).

## Converting between time zones

### 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). 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.