UTC (Coordinated Universal Time) is the global time standard from which all time zones are defined. It replaced GMT (Greenwich Mean Time) in 1972 as the primary time standard. Time zones are expressed as offsets from UTC — for example, Eastern Time is UTC-5 in winter and UTC-4 in summer (daylight saving time).

What time is it in New York when it is 9am in London?

During GMT/UTC offset periods (winter): New York is UTC-5, London is UTC+0, so New York is 5 hours behind London. If it is 9am in London, it is 4am in New York. During summer (when both observe daylight saving time), London moves to UTC+1 and New York to UTC-4, keeping a 5-hour difference.

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Quick Answer

There are 24 standard time zones and 38 distinct UTC offsets worldwide. UTC (Coordinated Universal Time) is the global reference — all zones are expressed as UTC+ or UTC-. New York is UTC-5 (EST) or UTC-4 (EDT). London is UTC+0 or UTC+1 during British Summer Time.

Understanding time zones

The Earth rotates 360 degrees every 24 hours, so each hour represents 15 degrees of longitude. In theory, this creates 24 time zones. In practice, countries set their own policies, creating 38 distinct UTC offsets. Some countries span multiple time zones (the US has 6) while others choose a single time zone for simplicity (China, despite spanning 5 geographical zones, uses UTC+8 everywhere).

Daylight saving time

About 70 countries observe daylight saving time (DST), which advances clocks by one hour in summer to extend evening daylight. The US and Canada move clocks forward on the second Sunday in March and back on the first Sunday in November. The EU observes DST from the last Sunday in March to the last Sunday in October. Many countries — including most of Asia, Africa, and now much of the US proposed legislation — do not observe DST.

Common UTC offsets

UTC-8: Pacific Time (US/Canada). UTC-7: Mountain Time. UTC-6: Central Time. UTC-5: Eastern Time. UTC+0: London/Dublin/Lisbon. UTC+1: Paris/Berlin/Rome/Madrid. UTC+2: Cairo/Athens/Johannesburg. UTC+3: Moscow/Nairobi/Istanbul. UTC+5:30: Mumbai/Delhi. UTC+8: Beijing/Singapore/Hong Kong/Perth. UTC+9: Tokyo/Seoul. UTC+10: Sydney/Melbourne. UTC+12: Auckland.

Frequently asked questions

How many time zones are there?

There are 24 standard time zones but 38 distinct UTC offsets in use worldwide, because some countries use 30- or 45-minute offsets (like India at UTC+5:30).

What is UTC?

UTC (Coordinated Universal Time) is the global time standard. All time zones are expressed as offsets from UTC. It replaced GMT in 1972 as the primary world time standard.

What time is it in New York when it's 9am in London?

New York is 5 hours behind London. If it's 9am in London, it's 4am in New York. This 5-hour difference applies year-round (both shift by one hour for daylight saving).

What is the difference between EST and EDT?

EST (Eastern Standard Time) is UTC-5, used in winter. EDT (Eastern Daylight Time) is UTC-4, used in summer when daylight saving time is in effect. Both are commonly called "Eastern Time (ET)".