# Time zone converter and meeting planner

> Time zone converter: turn a time in one zone into local time in up to 12 others, with daylight-saving changes applied and a meeting planner grid.

Interactive version: https://www.calcopenly.com/datetime/time-zone-converter
Subject: Date and time calculators

The converter reads the wall-clock time you enter in the source zone, subtracts that zone's UTC offset on that date to get a single instant, then adds each target zone's offset at the same instant. Offsets come from the IANA time zone database, so daylight-saving time is applied for the date you pick rather than for today.

The first worked example, 09:00 in New York on 15 January 2026, is 14:00 in London, 19:30 in India and 23:00 in Tokyo. The grid below the result shows all 24 hours of the source day in every zone, green where it falls inside Monday-to-Friday working hours (09:00 to 17:00 unless you change them), so a shared meeting slot is visible at a glance.

Some wall-clock times do not exist or happen twice. On 8 March 2026, 02:30 never occurs in New York and is read as 03:30; on 1 November 2026, 01:30 occurs twice and the first occurrence is used unless you choose the second.

## Inputs

- **Date**
- **Time**
- **From time zone** (options: This device's time zone, UTC, Honolulu, Anchorage, Los Angeles, Denver, Phoenix, Chicago, Mexico City, New York, Toronto, Bogotá, Halifax, St John's, Santiago, São Paulo, Buenos Aires, Reykjavík, London, Dublin, Lisbon, Lagos, Casablanca, Paris, Berlin, Madrid, Rome, Amsterdam, Stockholm, Oslo, Johannesburg, Cairo, Athens, Helsinki, Kyiv, Istanbul, Nairobi, Moscow, Riyadh, Tehran, Dubai, Karachi, India (Kolkata), Kathmandu, Dhaka, Bangkok, Jakarta, Singapore, China (Shanghai), Hong Kong, Manila, Perth, Seoul, Tokyo, Adelaide, Brisbane, Sydney, Auckland, Chatham Islands, Kiritimati)
- **Convert to**: IANA names (Europe/Paris) or cities from the list, separated by commas.
- **If the time happens twice (clocks going back)** (options: Use the first occurrence, Use the second occurrence)
- **Working day starts**
- **Working day ends**

## Results

- Converted time — main result
- UTC
- Offset of the source zone
- Daylight-saving check

## Formula

$$
t_{\text{UTC}} = t_{\text{local}} - \text{offset}_{\text{from}},\qquad t_{\text{target}} = t_{\text{UTC}} + \text{offset}_{\text{to}}(t_{\text{UTC}})
$$

## Worked examples

### 9 am in New York in January

- Date: 2026-01-15
- Time: 09:00
- From time zone: New York
- Convert to: Asia/Kolkata, Europe/London, Asia/Tokyo
- **Converted time: India (Kolkata) 19:30 · London 14:00 · Tokyo 23:00**
- **UTC: 2026-01-15T14:00:00Z**
- **Offset of the source zone: UTC−05:00**
- **Daylight-saving check: Exists once**
- Checked against: Python 3.8 dateutil.tz with system tzdata 2024a: datetime(2026,1,15,9,tzinfo=New_York).astimezone(...)

### A time skipped when New York springs forward

- Date: 2026-03-08
- Time: 02:30
- From time zone: New York
- Convert to: Europe/London, Asia/Kolkata
- **Converted time: London 07:30 · India (Kolkata) 13:00**
- **UTC: 2026-03-08T07:30:00Z**
- **Daylight-saving check: Skipped by a DST change**
- Checked against: Python 3.8 dateutil.tz: datetime_exists(2026-03-08 02:30 New_York) is False; resolve_imaginary gives 03:30 EDT = 07:30Z

### A repeated hour when New York falls back, second occurrence

- Date: 2026-11-01
- Time: 01:30
- From time zone: New York
- Convert to: Europe/London
- If the time happens twice (clocks going back): Use the second occurrence
- **Converted time: London 06:30**
- **UTC: 2026-11-01T06:30:00Z**
- **Offset of the source zone: UTC−05:00**
- **Daylight-saving check: Occurs twice (DST overlap)**
- Checked against: Python 3.8 dateutil.tz: datetime_ambiguous(2026-11-01 01:30 New_York) is True; fold=1 gives 01:30 EST = 06:30Z (fold=0 gives 05:30Z)

### Quarter-hour offsets

- Date: 2026-07-01
- Time: 12:00
- From time zone: Kathmandu
- Convert to: Pacific/Chatham, America/St_Johns
- **Converted time: Chatham Islands 19:00 · St John's 03:45**
- **UTC: 2026-07-01T06:15:00Z**
- **Offset of the source zone: UTC+05:45**
- Checked against: Python 3.8 dateutil.tz: Kathmandu +05:45, Chatham +12:45 (NZ winter), St John's −02:30 (summer time)

## Questions

### What is the difference between UTC and GMT?

On a clock they read the same, but UTC is an atomic time scale and GMT is a time zone. UTC is derived from International Atomic Time and kept within 0.9 seconds of the Earth's rotation by leap seconds; 27 have been added since 1972, the last on 31 December 2016. GMT is the UK's winter civil time; from late March to late October the UK uses BST, which is UTC+01:00.

### When do the clocks change for daylight saving time in 2026?

In the United States clocks go forward at 02:00 local time on the second Sunday in March and back on the first Sunday in November, the rule since 2007: 8 March and 1 November in 2026. The EU and UK change at 01:00 UTC on the last Sunday in March and October: 29 March and 25 October. Between 8 and 29 March, New York is 4 hours behind London instead of 5.

### Why use a zone name like America/New_York instead of EST?

An IANA name carries the zone's full history of offset and daylight-saving rules; an abbreviation is one fixed offset and is often ambiguous. EST means UTC−05:00, which is wrong for New York from March to November, when it is on EDT (UTC−04:00). IST can mean India (UTC+05:30), Irish or Israel Standard Time, and CST is both US Central and China Standard Time.

### Which time zones are offset by 30 or 45 minutes?

India uses UTC+05:30 all year, Iran UTC+03:30, Afghanistan UTC+04:30 and Myanmar UTC+06:30. Nepal is at UTC+05:45 and the Chatham Islands at UTC+12:45, or UTC+13:45 in the southern summer. Newfoundland is at UTC−03:30 in winter and South Australia at UTC+09:30 in winter, and both move by an hour for daylight saving.

### How many working hours do London and New York share?

Three hours for most of the year: 09:00 to 17:00 in New York is 14:00 to 22:00 in London, so the shared window is 14:00 to 17:00 London time. It widens to four hours in the weeks when only the US has changed its clocks, such as 8 to 29 March 2026. Tokyo's 09:00 to 17:00 does not overlap either city's working day.

### How accurate is the time zone converter and meeting planner?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 4 worked examples whose answers come from independent sources; for example, “9 am in New York in January” is checked against Python 3.8 dateutil.tz with system tzdata 2024a: datetime(2026,1,15,9,tzinfo=New_York).astimezone(...).

### Where does the method come from?

IANA Time Zone Database; ECMA-402 Intl.DateTimeFormat (timeZone option); TC39 Temporal — time zones and resolving ambiguity.

## Sources

- [IANA Time Zone Database](https://www.iana.org/time-zones)
- [ECMA-402 Intl.DateTimeFormat (timeZone option)](https://tc39.es/ecma402/#datetimeformat-objects)
- [TC39 Temporal — time zones and resolving ambiguity](https://tc39.es/proposal-temporal/docs/timezone.html)