UTCoffsets

UTC offsets are the differences between local time and Coordinated Universal Time (UTC). They are typically written as UTC followed by a plus or minus sign and an hour (and sometimes minute) offset, for example UTC+01:00 or UTC-05:00. A local time can be described as UTC time plus its offset.

Offsets are the basis for time zones. Standard offsets designate a region’s nominal time, while some regions adjust their offset during part of the year. For instance, Central European Time is UTC+01:00, and its daylight saving counterpart is UTC+02:00. Eastern Time in the United States is UTC-05:00 in standard time and UTC-04:00 during daylight saving time. Some regions use non-integer offsets, such as India at UTC+05:30 or Nepal at UTC+05:45.

Daylight saving time (DST) is a common source of offset changes. In areas that observe DST, clocks are set forward by one hour in a warm period and set back in a cooler period, resulting in different offsets at different times of the year. Not all places observe DST, and some have experimented with permanent standard time or permanent daylight time, leading to permanent offsets that do not change seasonally.

Offsets can be non-hourly or non-standard in historical or political contexts. Examples include UTC+05:45 in Nepal and UTC-03:30 in Newfoundland. Since time laws can change, offsets may shift when governments adjust regulations.

In computing and data exchange, offsets are tracked with time zone databases that encode the rules for each region. Local time is often expressed as UTC plus its current offset, and time conversions must account for DST and other regulatory changes to avoid ambiguity.

See also: time zone, daylight saving time, IANA time zone database.