# Đổi múi giờ và lên lịch họp

> Đổi giờ của một múi giờ sang giờ địa phương của tối đa 12 múi giờ khác, áp dụng giờ mùa hè và cung cấp bảng lên lịch họp.

Phiên bản tương tác: https://www.calcopenly.com/vi/datetime/time-zone-converter
Chủ đề: Máy tính ngày và giờ

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.

## Dữ liệu đầu vào

- **Date**
- **Thời gian**
- **From time zone** (lựa chọn: 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)** (lựa chọn: Use the first occurrence, Use the second occurrence)
- **Working day starts**
- **Working day ends**

## Kết quả

- Converted time — kết quả chính
- UTC
- Offset of the source zone
- Daylight-saving check

## Công thức

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

## Ví dụ có lời giải

### 9 am in New York in January

- Date: 2026-01-15
- Thời gian: 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**
- Nguồn đối chiếu: 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
- Thời gian: 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**
- Nguồn đối chiếu: 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
- Thời gian: 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)**
- Nguồn đối chiếu: 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
- Thời gian: 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**
- Nguồn đối chiếu: Python 3.8 dateutil.tz: Kathmandu +05:45, Chatham +12:45 (NZ winter), St John's −02:30 (summer time)

## Câu hỏi

### 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.

### “Đổi múi giờ và lên lịch họp” chính xác đến mức nào?

Độ chính xác phụ thuộc vào dữ liệu nhập và giả định của phương pháp. Phép tính thập phân dùng 50 chữ số có nghĩa, nhưng ước lượng, phương pháp số và dữ liệu nguồn có thể kém chính xác hơn; làm tròn khi hiển thị không loại bỏ các giới hạn đó. Ví dụ có lời giải đã đối chiếu với nguồn độc lập: 4. Ví dụ, “9 am in New York in January” được kiểm tra bằng Python 3.8 dateutil.tz with system tzdata 2024a: datetime(2026,1,15,9,tzinfo=New_York).astimezone(...).

### Phương pháp này lấy từ đâu?

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

## Nguồn

- [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)