PST, PDT and IST: How Time Zones and Daylight Saving Work
UTC offsets, why the US–India gap flips between 12½ and 13½ hours, the weeks the US and Europe disagree, IANA zone names, and when working hours overlap.
Productivity···8 min read
If you work between California and India, you've probably noticed that the weekly call moves by an hour twice a year, even though nobody changed the invite. In March, 8:00 am in San Francisco stops being 9:30 pm in Bengaluru and becomes 8:30 pm. In November it moves back.
Nothing is wrong with the calendar. The two places keep time differently, and once you see how, the moves are easy to predict. This guide covers UTC offsets, what PST, PDT and IST actually mean, why the US–India gap changes, the odd weeks when the US and Europe are out of step, and how much of a working day different teams share.
Everything is an offset from UTC
The world's clocks are set from one reference, Coordinated Universal Time (UTC), which the International Bureau of Weights and Measures (BIPM) maintains from atomic clocks around the world. Local time in any place is UTC plus or minus a fixed amount, its offset:
| Name | Where | Offset |
|---|---|---|
| PST, Pacific Standard Time | US West Coast, in winter | UTC−8 |
| PDT, Pacific Daylight Time | US West Coast, in summer | UTC−7 |
| EST / EDT, Eastern Time | US East Coast, winter / summer | UTC−5 / UTC−4 |
| GMT / BST, UK time | London, winter / summer | UTC+0 / UTC+1 |
| CET / CEST, Central European Time | Berlin, Paris, winter / summer | UTC+1 / UTC+2 |
| IST, India Standard Time | All of India, all year | UTC+5:30 |
Converting between any two places takes two steps: go to UTC, then out to the other place. For 9:00 am PST: add 8 hours to reach UTC (17:00), then add 5½ hours to reach India (22:30, or 10:30 pm IST).
India's offset includes a half hour, and it covers the whole country. India has used UTC+5:30 since 1906 and hasn't moved its clocks since October 1945, according to the IANA time zone database's history of Indian time.
PST is only half the year
"PST" is one of the most misused abbreviations in scheduling. Strictly, Pacific Standard Time is only in force from early November to mid-March. For the rest of the year, California, Washington and Oregon are on Pacific Daylight Time, an hour ahead.
US federal law (15 U.S.C. § 260a) sets the dates: daylight saving time starts at 2:00 am on the second Sunday of March and ends at 2:00 am on the first Sunday of November. That schedule came in with the Energy Policy Act of 2005. A few places opt out. NIST lists Hawaii, most of Arizona, Puerto Rico, Guam, American Samoa and the US Virgin Islands as not observing daylight saving time.
So "the meeting is at 9 am PST" in July is technically an hour off. If you aren't sure which half of the year a date falls in, write PT (Pacific Time), and the reader's calendar will apply whichever one is in force.
Why the US–India gap changes in March and November
India never changes its clocks. The US West Coast does. So the gap between them changes whenever California does:
| Period (2026) | California | India | India is ahead by |
|---|---|---|---|
| 1 Jan – 7 Mar | PST, UTC−8 | IST, UTC+5:30 | 13 h 30 min |
| 8 Mar – 31 Oct | PDT, UTC−7 | IST, UTC+5:30 | 12 h 30 min |
| 1 Nov – 31 Dec | PST, UTC−8 | IST, UTC+5:30 | 13 h 30 min |
For a meeting set in California time, Indian attendees see it move an hour earlier in March (8:00 am PDT is 8:30 pm IST) and an hour later in November (8:00 am PST is 9:30 pm IST). A meeting set in India time does the opposite for the Americans.
The US East Coast follows the same dates, so New York to India is 10 h 30 min in winter and 9 h 30 min in summer.
In 2027 the switch dates are 14 March and 7 November. You can see the difference on either side of this year's change: 8:00 am on 27 October is PDT, and 8:00 am on 3 November is PST.
The weeks when the US and Europe disagree
Europe also has daylight saving time, but on different dates. In the EU (and in the UK, which follows the same rule), summer time starts on the last Sunday of March and ends on the last Sunday of October, at 01:00 UTC everywhere at once.
Because the US switches earlier in spring and later in autumn, there are a few weeks each year when the usual gap between them shrinks by an hour. In 2026 those weeks are 8–28 March and 25–31 October:
| Gap | Usually | In those weeks |
|---|---|---|
| New York to London | 5 h | 4 h |
| Los Angeles to Berlin | 9 h | 8 h |
In 2027 those weeks are 14–27 March and 31 October to 6 November. A standing call at 9:00 am in New York, normally 2:00 pm in London, lands at 1:00 pm in those weeks. Check 9:00 am EDT on 27 October.
The gap between Europe and India changes too, because Europe moves and India doesn't. London is 5 h 30 min behind India in winter and 4 h 30 min behind in summer; Berlin is 4 h 30 min and 3 h 30 min behind.
Abbreviations are ambiguous; zone names aren't
IST can mean India Standard Time, Irish Standard Time (Ireland's summer time) or Israel Standard Time. CST is US Central Standard Time and also China Standard Time. The maintainers of the IANA time zone database, the shared record of every region's clock rules that computers rely on, call these abbreviations "ambiguous in practice" and recommend numeric offsets instead.
Software avoids the problem with IANA zone names, written as Area/Location:
America/Los_Angelesfor US Pacific TimeAmerica/New_Yorkfor US Eastern TimeEurope/Londonfor UK timeAsia/Kolkatafor IndiaAmerica/Phoenixfor Arizona, which stays on UTC−7 all year
A zone name stands for a place's whole history of rules, not one offset. America/Los_Angeles knows that 27 October 2026 is UTC−7 and 3 November 2026 is UTC−8. The database names zones after cities rather than countries "so that any future changes do not split individual locations into different timezones".
When you write a time down for people in other countries, two habits remove most mistakes:
- Give the date as well as the time. The same 8:00 am in California has a different gap to India in October than in November, and late-evening times in one place are often the next day in another.
- For anything exact, give the offset, as in RFC 3339 timestamps:
2026-11-03T08:00:00-08:00means 8:00 am at UTC−8, with no room for doubt.
The clock-change night itself
On the night the clocks move, one hour is skipped or repeated. In the US, the spring change goes straight from 1:59 am to 3:00 am, so 2:30 am doesn't happen that day. In November, 1:00–1:59 am happens twice. Avoid scheduling anything automatic (backups, reminders, payments) between 1 am and 3 am local time on those Sundays.
How much of the working day overlaps
If both sides keep a 9:00 am to 5:00 pm day, here's how many hours they share:
| Pair | Overlap in winter | Overlap in summer |
|---|---|---|
| California and India | none | none |
| New York and India | none | none |
| London and India | 2 h 30 min | 3 h 30 min |
| Berlin and India | 3 h 30 min | 4 h 30 min |
| New York and London | 3 h | 3 h |
| New York and Berlin | 2 h | 2 h |
Summer here means late March to late October, when the US and Europe are both on summer time. In the mismatch weeks above, New York and London share 4 hours and New York and Berlin share 3.
US–India teams have no shared office hours at all, so someone stretches their day. Two windows work:
- US morning, India evening. 7:30 am PDT is 8:00 pm IST; 8:00 am PST is 9:30 pm IST. From New York, 9:00 am EDT is 6:30 pm IST, and 9:00 am EST is 7:30 pm IST.
- US evening, India morning. 6:30 pm PDT on Monday is 7:00 am IST on Tuesday, before the Indian working day starts.
Try it
The PST to IST converter works out the right offset for the date you pick, labels it PST or PDT, and shows how the two working days overlap. The world clock keeps the cities you work with side by side, and the link saves your list. There are pages for the other common pairs too, such as EST to IST, CET to IST and IST to PST.
Sources
- 15 U.S.C. § 260a, Advancement of time or changeover dates (US daylight saving time). Cornell LII
- NIST. Daylight Saving Time (DST): rules, history and places that don't observe it. nist.gov
- Directive 2000/84/EC of the European Parliament and of the Council on summer-time arrangements. EUR-Lex
- GOV.UK. When do the clocks change? gov.uk
- IANA time zone database: Theory and pragmatics (zone names and abbreviations). iana.org
- IANA time zone database, file
asia(India's time since 1854). GitHub: eggert/tz - BIPM. Time metrology (UTC). bipm.org
- RFC 3339, Date and Time on the Internet: Timestamps. rfc-editor.org
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