Scheduling a call with a team in another country, figuring out when a service update will be released in your region, or finding the local time at your travel destination—all of these require converting between time zones. Let’s look at how it works.
What are time zones and UTC?
UTC (Coordinated Universal Time) is the international time standard, based on atomic clocks that are not adjusted seasonally. It is the reference point for all other time zones.
GMT (Greenwich Mean Time) is the historical standard, based on the Sun’s position relative to the Greenwich meridian. In everyday use, GMT and UTC are often used as synonyms, although UTC is technically more precise.
Each time zone is defined by its offset from UTC:
| City | Zone | Offset |
|---|---|---|
| London | GMT/UTC | UTC+0 (winter) / UTC+1 (summer) |
| Moscow | MSK | UTC+3 |
| Dubai | GST | UTC+4 |
| Beijing | CST | UTC+8 |
| Tokyo | JST | UTC+9 |
| New York | EST/EDT | UTC-5 (winter) / UTC-4 (summer) |
| Los Angeles | PST/PDT | UTC-8 (winter) / UTC-7 (summer) |
Popular time zone pairs
Moscow — New York
In winter, the difference is 8 hours (MSK = UTC+3, EST = UTC-5). When it is noon in Moscow, it is 4:00 a.m. in New York.
In summer, the US switches to daylight saving time, reducing the difference to 7 hours. Russia abolished daylight saving time in 2014.
Moscow — London
In winter: 3 hours (UTC+3 vs. UTC+0). In summer: 2 hours (the UK switches to BST = UTC+1).
Moscow — Tokyo
The difference is a constant 6 hours (UTC+3 vs. UTC+9). Japan does not observe daylight saving time.
Daylight saving time: when and where
Around 70 countries observe Daylight Saving Time (DST). The idea is to move clocks forward by 1 hour in spring and back again in autumn so that daylight hours align with working hours.
- US: the 2 Sunday in March (forward) and the 1 Sunday in November (back)
- European Union: the last Sunday in March and the last Sunday in October
- Australia: the first Sunday in October (the seasons are reversed in the Southern Hemisphere)
Russia, China, Japan, India, and most Asian countries do not observe daylight saving time.
IANA database and browser APIs
The world’s time zones are described in the IANA Time Zone Database (tzdata), an open database that is updated whenever countries change their laws. All modern operating systems and browsers use it.
In JavaScript, time zones are available through Intl.DateTimeFormat:
const formatter = new Intl.DateTimeFormat('ru-RU', {
timeZone: 'America/New_York',
hour: '2-digit', minute: '2-digit'
});
console.log(formatter.format(new Date())); // текущее время в Нью-ЙоркеIANA identifiers look like Europe/Moscow, America/New_York, Asia/Tokyo; they account for historical changes and daylight saving time automatically.
Tips for planning international meetings
- Specify the time with an explicit time zone: “15:00 UTC” instead of “15:00”
- Account for daylight saving time changes—the time difference can change during the transition periods
- Use Unix timestamps to store time in databases—they are independent of time zones
If you need to set up scheduled tasks across time zones, use the cron expression parser.
Tool on reChecker
The reChecker time zone converter lets you select any two time zones from the IANA database and convert a specified time. It supports all current time zones around the world, accounting for daylight saving time.