how do GMT/ UTC (+-) works?


For example, if it’s 06:00 (UTC+8), how would I convert it? How does the +8 part works?

In: 1

UTC is the “reference” time zone, located in the United Kingdom (this used to be Greenwich Mean Time, or GMT). So if your time zone is UTC+8, that means that all your times are 8 hours ahead of UTC. 1200/Noon for you would be equivalent to 0400/4 am GMT.

GMT (Greenwhich Mean Time) is the UK standard timezone.

UTC is Universal Coordinated Time. It is a standard time that is the basis of all other time zones.

The + or – tells you to add or subtract the zone value from UTC.

UTC+8 would mean the current UTC time with 8-hours added to it.

For example, at the time of posting this, it is 0645 UTC. Due to daylight savings, GMT is currently 0745. (This means it is UTC+1)

In New York (UTC-5) it is 0245 because we subtract 5 hours from UTC.

2am Central Time is -5 hours gmt -5 so at 2am central time you add 5 to have GMT 0 making gmt 0 7am and +8 would be 3pm.

6 – 8 = 22 in modulo 24, i.e. wrapping around at 24 (never using the number 24 itself).

So the location where it is 06:00 is 8 hours ahead (east) of GMT / UTC 0, Greenwich (London), England.

> The earth is divided into 24 time zones, -11 to +12. Each time zone is 1 hour long, or 15° wide in longitude. Greenwich England is, by definition, in the middle of Time Zone 0, the prime meridian. UTC time is the local time at Greenwich England. Time in other locations will be the UTC time hour plus or minus the local Time Zone. Daylight savings time adds 1 hour to the local standard (real) time. -wikipedia

Clearly the international dateline is very roughly opposite GMT so when you cross that you jump forward by a day if traveling west from GMT.

UTC is todays standard time reference. Before it was GMT, these two are theoretically slightly different (few seconds) but de facto the same. UTC is today the time standard and GMT is a time zone UTC (0). GMT was established by the British for synchronization of the train schedule, before that 12 o’clock was always local sun high

The conversion factor is often only relevant if you travel between zones (or make other contact across zones ie via internet)

If you are in UTC (+8) (Asia, ie China, Malaysia and Singapore among more), the time at UTC 1200, it is 2000 in Beijing.

If you work at sea like me, we often want to calculate what is the UTC time, we use Zone Time (ZT), here the calculation is the other way around. So 1200 ZT(-8) (again China etc), is UTC 0400

Many android phone clocks have world time tab where alarms, timers and stopwatch is. It helps when coordinating not only time but the day. Helps for pogo raids US to Australia.

UTC is technically not a timezone, but a timestandard. It is the basis used for all other timezones.


Due to the rotation of the earth, depending on the location you are, it might be night, day, morning, …As traditionally, noon is when the sun is at the higest position in the sky and this will not happen simultaneously for different locations on earth. Different timezones were cretated so noon is always approximately at 12:00h. (Let’s ignore summertime for a moment.)

GMT is a timezone that doesn’t have any offset with UTC, so GMT is “UTC+0”

I’m in Belgium, currently on CEST (Central European Summer Time), which is “UTC+2”.

The +/- digit, is the number of hours difference with UTC, so you need to add/subtract that number of hours.
This means if it’s 8:00 UTC, it’s 10:00 CEST.

If you are in UTC+8, it means 8:00 UTC is 16:00h your timezone. (4PM)

If you want to calculate the timedifference between different timezones, you can use the offset they have between UTC.
I’m UTC+2, your are UTC+8, we have a 6 hour time difference and you are ahead of me. 10:00h my time, is 16:00h your time.

06:00 UTC+8, is 22:00 UTC+0/GMT (previous day), or 00:00 CEST/UTC+2 (midnight)