Frequently Asked Questions

What is a timezone?

A timezone is a region that observes a uniform standard time. Timezones are defined by their offset from Coordinated Universal Time (UTC), the world's primary time standard. For example, Central European Time is UTC+1. There are currently around 395 timezones in use worldwide. Source: IANA Time Zone Database (iana.org/time-zones).

What is UTC and why is it the world's reference time?

Coordinated Universal Time (UTC) is the primary time standard that regulates clocks worldwide. It is based on International Atomic Time (TAI) with leap seconds added to stay aligned with Earth's rotation. UTC replaced Greenwich Mean Time (GMT) as the official reference standard in 1972. Sources: NIST (time.gov), BIPM.

How does Daylight Saving Time work?

Daylight Saving Time (DST) shifts clocks forward, usually by one hour in spring ('spring forward') and back in autumn ('fall back'), to extend evening daylight. In the U.S., DST runs from the second Sunday in March to the first Sunday in November. Not all regions observe DST — Arizona (except the Navajo Nation) and Hawaii stay on standard time year-round. Sources: timeanddate.com, NIST.

What is the time difference between New York and London?

London is normally 5 hours ahead of New York. During parts of March and October–November, when the U.S. and the UK change their clocks on different dates, the difference temporarily becomes 4 hours. TZSync calculates the correct difference for any date automatically.

What is Indian Standard Time (IST)?

India observes a single timezone: Indian Standard Time, UTC+5:30. India does not observe Daylight Saving Time. IST is 30 minutes ahead of Pakistan (UTC+5) and 30 minutes behind Myanmar (UTC+6:30). Source: Government of India / time.gov.in.

Why do some timezones have half-hour or 45-minute offsets?

Many regions offset their clocks by 30 or 45 minutes from UTC to better match solar time for their longitude. Examples include India (UTC+5:30), Nepal (UTC+5:45), Newfoundland (UTC-3:30), and the Chatham Islands of New Zealand (UTC+12:45). TZSync handles these offsets correctly.

What is the International Date Line?

The International Date Line (IDL) is an imaginary line near the 180° meridian where the calendar date changes. Crossing it westward adds a day; crossing it eastward subtracts a day. This is why Tokyo is 'tomorrow' relative to Hawaii even though they are only about 7–8 hours apart in solar terms. Sources: IANA tzdb, NOAA.

How many timezones does the world have?

The world uses roughly 395 distinct time zones, including those with 30- and 45-minute offsets. The theoretical basis is 24 hourly zones, but political and geographic decisions have fragmented them. Source: IANA Time Zone Database (tzdb), maintained by Paul Eggert and contributors.

What timezone is used in China?

China officially uses a single timezone: China Standard Time (CST), UTC+8, for the entire country, despite spanning roughly five geographic time zones. This policy has been in place since 1949. Xinjiang residents informally use UTC+6 in daily life. Source: timeanddate.com.

How do I schedule a meeting between the US and India?

India is 9.5 to 13.5 hours ahead of the contiguous U.S. A practical overlap window is 7:30–10:00 AM Eastern Time (5:00–7:30 PM India), or 6:30–9:00 PM Eastern Time (7:00–9:30 AM India next day). Use TZSync's converter to find the exact overlap for your date, since DST shifts the gap by an hour for part of the year.

What is Unix timestamp?

A Unix timestamp is the number of seconds elapsed since January 1, 1970 at 00:00:00 UTC (the Unix epoch). It is timezone-independent, which is why computers use it internally and convert to local time only for display. TZSync performs the same conversion for human-readable times.

Does my phone automatically adjust for other timezones when I travel?

Modern smartphones adjust automatically when 'Set Automatically' (network-provided time) is enabled. Airline crew and frequent travelers sometimes disable this and use a world clock app like TZSync to track home time. TZSync works in any browser without installation.

What is the difference between GMT and UTC?

GMT (Greenwich Mean Time) is a timezone based on solar time at the Royal Observatory, Greenwich. UTC is an atomic-time standard that replaced GMT as the world reference in 1972. In everyday use they are effectively interchangeable (UTC+0 = GMT), but only UTC is the official standard. Source: BIPM, NIST.

What is ISO 8601 date-time format?

ISO 8601 is the international standard for representing dates and times, e.g. 2026-08-18T14:30:00+05:30. The suffix is the UTC offset, and 'Z' denotes UTC itself. It is widely used in APIs, databases, and flight schedules because it is unambiguous. Source: ISO 8601-1:2019.

Is TZSync really free?

Yes. TZSync's converter, world clocks, and timezone reference pages are completely free with no ads in the tools themselves. Optional paid plans (Silver, Golden, Diamond) add features like more saved cities, device sync, and priority support. See the pricing page.