Time Zones, UTC and Daylight Saving, Explained
UTC vs GMT, offsets such as +05:30 and +05:45, IANA zone names, why EST and IST are ambiguous, daylight saving shifts, and using a time zone converter well.
By Udit Agarwal · Updated October 17, 2026
A time zone is a region that keeps the same clock time, defined by its offset from Coordinated Universal Time (UTC) and by rules for when that offset changes, such as daylight saving time. New York is UTC−5 in winter and UTC−4 in summer, while India stays at UTC+5:30 all year. Most time zone mistakes come from treating an offset, an abbreviation and a zone as the same thing.
People describe a zone with a single number, such as "UTC+1", but a zone is really a place with a history. Europe/London is UTC+0 in winter and UTC+1 in summer, and its rules have changed before. That is why software uses named zones rather than fixed offsets: the name stands for every offset the place has used and when it switched.
Those rules live in the IANA time zone database, also called the tz or Olson database. Operating systems, browsers and programming languages ship a copy, and it is updated several times a year as governments change their clocks.
UTC and GMT show the same clock time today, but they are different things:
The practical difference: London is not always on GMT, because from late March to late October the UK uses British Summer Time, UTC+1. When you mean the universal reference, say UTC. Some formats still write GMT where they mean UTC: HTTP dates and JavaScript's toUTCString() both end in "GMT".
An offset is the difference from UTC at a given moment, written as +05:30 or −04:00. Offsets run from UTC−12:00 to UTC+14:00, and not all of them are whole hours:
| Place | IANA zone | Offset | Daylight saving |
|---|---|---|---|
| India | Asia/Kolkata |
+05:30 | none |
| Nepal | Asia/Kathmandu |
+05:45 | none |
| Iran | Asia/Tehran |
+03:30 | none |
| South Australia | Australia/Adelaide |
+09:30 | +10:30 in summer |
| Newfoundland, Canada | America/St_Johns |
−03:30 | −02:30 in summer |
| Chatham Islands, New Zealand | Pacific/Chatham |
+12:45 | +13:45 in summer |
| Kiribati (Line Islands) | Pacific/Kiritimati |
+14:00 | none |
One trap in the database itself: names such as Etc/GMT+5 use the old POSIX sign convention, so Etc/GMT+5 is five hours behind UTC. Prefer place names.
IANA names follow an Area/Location pattern, such as America/New_York, Europe/Berlin or Asia/Tokyo. The location is usually the most populous city of a region whose clocks have agreed since 1970. Renamed zones keep their old names as aliases, so you will still meet Asia/Calcutta for Asia/Kolkata and Europe/Kiev for Europe/Kyiv.
Abbreviations are another matter. They are not standardized, and several are shared:
| Abbreviation | Can mean | Offset |
|---|---|---|
| IST | India Standard Time | +05:30 |
| IST | Irish Standard Time (summer) | +01:00 |
| IST | Israel Standard Time | +02:00 |
| CST | Central Standard Time (North America) | −06:00 |
| CST | China Standard Time | +08:00 |
| CST | Cuba Standard Time | −05:00 |
| BST | British Summer Time | +01:00 |
| BST | Bangladesh Standard Time | +06:00 |
Abbreviations also blur daylight saving. "9:00 EST" written in July is ambiguous: the writer probably means New York time, which is EDT (UTC−4) in summer, but EST strictly means UTC−5. "New York time" is safer, and adding the UTC time removes any doubt. In software, store the IANA name.
Daylight saving time moves clocks forward an hour in spring and back in autumn, so a zone's offset changes twice a year. Regions change on different dates:
| Region | Clocks go forward | Clocks go back | 2026 dates |
|---|---|---|---|
| Most of the US and Canada | 2nd Sunday in March, 2:00 | 1st Sunday in November, 2:00 | 8 March, 1 November |
| UK and EU | last Sunday in March, 01:00 UTC | last Sunday in October, 01:00 UTC | 29 March, 25 October |
| Sydney and Melbourne | 1st Sunday in October | 1st Sunday in April | 4 October, 5 April |
| New Zealand | last Sunday in September | 1st Sunday in April | 27 September, 5 April |
Many places do not change their clocks at all, including India, China, Japan, Brazil, most of Mexico, most of Africa, Hawaii and most of Arizona.
Mismatched dates shift the gaps between zones. Between 25 October and 1 November 2026, London has moved back but New York has not, so New York is 4 hours behind London instead of 5; the same happens from 8 to 29 March. Across hemispheres the effect doubles: London is 9 hours behind Sydney in July, 10 in mid-October and 11 in January.
Clock changes also create local times that never happen or happen twice. In New York, 2:30 on 8 March 2026 does not exist, and 1:30 on 1 November 2026 occurs at 05:30 UTC and again at 06:30 UTC.
A recurring meeting is pinned to one zone, usually the organizer's, and everyone else's local time moves when zones change on different dates. Here is a weekly Monday call at 9:00 New York time through autumn 2026:
| Monday | New York | London | Bengaluru | Tokyo |
|---|---|---|---|---|
| 19 October | 09:00 (UTC−4) | 14:00 | 18:30 | 22:00 |
| 26 October | 09:00 (UTC−4) | 13:00 | 18:30 | 22:00 |
| 2 November | 09:00 (UTC−5) | 14:00 | 19:30 | 23:00 |
London's time jumps for one week and returns, while Bengaluru and Tokyo move an hour later until March.
Some combinations have no comfortable slot at all. With a 9:00 to 18:00 working day, San Francisco and Bengaluru never overlap in November: 9:00 in San Francisco is 22:30 in Bengaluru, so someone has to stretch, and a fair team rotates who. A few habits prevent most mistakes:
Store past events and exact instants in UTC. A log line, an order or a login happened at one instant, so store it as a Unix timestamp or a UTC date-time and convert to local time only for display. The Unix Timestamp Converter shows any epoch value in UTC and in your local time. In PostgreSQL, timestamptz stores an instant this way, while plain timestamp stores a wall-clock reading with no zone.
Store future local events as a local time plus an IANA zone. "9:00 every Monday in New York" is a rule about New York's clocks, and the UTC instant depends on daylight saving rules that can change before the date arrives; Mexico dropped daylight saving for most of the country in 2022, for example. If an app works out the UTC time on 10 November 2026, when New York is at UTC−5, it stores 14:00 UTC for a meeting on 15 March 2027. By then New York is on daylight time, and 9:00 local is 13:00 UTC. Storing 09:00 with America/New_York and converting when needed gets it right. The Unix timestamps guide covers the UTC half in more depth.
The World Clock & Time Zone Converter lines up as many places as you like on one 24-hour grid, each row a city and each column the same moment. Search by city, country or abbreviation: typing IST lists the Indian cities alongside Dublin and Jerusalem, each with its current offset, so you choose a place rather than trusting the letters. Click a half-hour slot or drag across the grid to select a meeting, and every row shows its local time, its GMT offset and a +1d or −1d marker when the date differs. Work hours, 9:00 to 18:00, are highlighted and night hours dimmed, so overlaps stand out.
Pick any date to plan ahead: offsets come from your browser's built-in time zone database, so daylight saving is applied for that date. You can switch between 12- and 24-hour clocks, make any row the reference and copy every converted time at once. Your cities are remembered on your device and nothing is uploaded. Every zone your browser knows is searchable, though some browsers list older spellings, such as Katmandu for Kathmandu.
Is UTC the same as GMT? For everyday purposes they show the same time. UTC is the atomic-clock standard that computers and aviation use, while GMT is a time zone used by the UK in winter and by some countries all year. The UK moves to UTC+1 in summer, so London is not always on GMT.
What is the difference between EST and EDT? EST, Eastern Standard Time, is UTC−5 and applies in winter. EDT, Eastern Daylight Time, is UTC−4 and applies from the second Sunday in March to the first Sunday in November. "Eastern Time" covers both.
Why does India have a 30-minute offset? India Standard Time is based on the meridian at 82.5° east, near the middle of the country. At 15 degrees per hour that gives 5 hours 30 minutes ahead of UTC. India uses one zone for the whole country, with no daylight saving.
Which places use a 45-minute offset? Nepal, at UTC+5:45, is the best-known example. New Zealand's Chatham Islands use UTC+12:45, or +13:45 in summer.
When do the clocks change in 2026? In most of the US and Canada, clocks go forward on 8 March and back on 1 November. In the UK and the EU they go forward on 29 March and back on 25 October. Parts of Australia and New Zealand go forward in late September or early October.
What time zone should I store in a database? Store instants, such as when an order was placed, in UTC. For future events set in local time, store the local time together with an IANA zone name such as Europe/Berlin, because daylight saving rules can change before the event happens.
Does the time zone converter handle daylight saving? Yes. The ArrayKit converter uses your browser's built-in time zone database, so each city's offset is the one in force on the date you pick, including the weeks when regions are out of step.
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