UTC, Coordinated Universal Time, is the reference every time zone is measured from. GMT, Greenwich Mean Time, is older and today means two different things: the time zone the UK uses in winter, and a casual synonym for UTC. For converting times the two are interchangeable; the difference between them is a fraction of a second that only matters to astronomers. The third use is the label on phones and computers, where "GMT+9" simply means nine hours ahead of UTC.
The trap is the first meaning. "London is on GMT" is true only from the last Sunday in October to the last Sunday in March. In summer London is on British Summer Time, UTC+1, and a converter that treats London as UTC all year is wrong for seven months of it.
An offset tells you how far a zone is from UTC. UTC+9 means the local clock reads nine hours later than UTC; UTC−7 means seven hours earlier. To go from local time to UTC, reverse the sign; to go from UTC to local, apply it. The difference between two zones is the difference of their offsets: Seoul (UTC+9) minus Vancouver in summer (UTC−7) is 16 hours, and in winter (UTC−8) it is 17.
A worked example. 9:00 AM in Seoul on Thursday 15 October 2026 is 00:00 UTC that day. London is UTC+1 on that date, so it is 1:00 AM Thursday there. Vancouver is UTC−7, so it is 5:00 PM on Wednesday — the date moves back because you are subtracting past midnight. Run the same conversion for 10:00 AM in Vancouver on 20 January 2027 and the offsets have changed: Vancouver is UTC−8, so it is 18:00 UTC, which is 6:00 PM in London (UTC+0) and 3:00 AM on Thursday in Seoul.
Not every offset is a whole hour. India is UTC+5:30, Newfoundland is UTC−3:30, Adelaide is UTC+9:30 in winter and UTC+10:30 in summer, and Nepal is UTC+5:45. A converter handles these; mental arithmetic usually does not.
Daylight saving time moves the clock forward one hour in spring and back in autumn, and the dates differ by region. These are the changes for the next two years:
Because the regions change on different dates, there are windows every year when the usual gap between two cities is off by an hour. From 8 to 29 March 2026 North America has moved forward and Europe has not, so New York and London are four hours apart instead of five. From 25 October to 1 November 2026 it happens again in the other direction. The same two windows recur in 2027 from 14 to 28 March and from 31 October to 7 November. And every time Europe or North America changes, its gap to Seoul, Tokyo and Singapore changes by an hour, because those cities never move.
America/Vancouver, rather than three letters.Open the time zone converter and turn off "Using Current Time" so you can enter a date. Pick the From and To zones from the list of 26; Seoul sits under Japan Standard Time and Vancouver under Pacific Time, since each keeps the same clock as the listed city all year. Enter the date and time, and read the result with its summary line, for example "09:00 JST = 17:00 PDT (Previous day)". The offsets shown in the dropdown follow the date you entered, so Eastern Time reads −04:00 for a July date and −05:00 for a January one. The swap button reverses the conversion, and the Common Business Hours panel shows 9 AM to 5 PM New York on your chosen date in London, Tokyo and Sydney.
For what time it is in several cities right now, the world clock is quicker than converting. For finding a slot that suits several people, the meeting planner ranks every half hour of the day; this worked example for Vancouver, London and Seoul shows how. The broader background on how zones came to be and how software should store times is in the practical guide to time zones.
Open the free time zone converter — enter the date of the meeting, not today, and the daylight saving rules take care of themselves.
For converting times, yes: both are offset zero and labels like GMT+9 mean nine hours ahead of UTC. The one thing to remember is that the UK itself is only on GMT in winter; from late March to late October London is on British Summer Time, UTC+1.
Because New York observes daylight saving time. It is UTC−4 (EDT) from the second Sunday in March to the first Sunday in November and UTC−5 (EST) the rest of the year, and the converter applies the rule for the date you enter.
South Korea, Japan, China, India and Singapore never change, and most of Mexico stopped in 2022. Within countries that do, Arizona (except the Navajo Nation), Hawaii, Queensland, Western Australia and the Northern Territory stay on standard time all year.
Reverse the source offset to get UTC, then apply the target offset. 9:00 AM in Seoul (UTC+9) is 00:00 UTC; add Vancouver’s summer offset of −7 and you get 5:00 PM the previous day. Check the date whenever the result crosses midnight.