What is the difference between UTC and GMT?

Question

I have a few queries regarding the Time zones:

1. Can the time be captured in UTC alone?
2. Is UTC -6 and GMT -6 the same, and does that mean it is US local time?
3. Say, I have UTC time as "02-01-2018 00:03" does that mean my US local time is "01-01-2018 18:00"?

I have searched on Wikipedia and many related websites but haven't found a relevant explanation.

Solution 1

There is no time difference between Coordinated Universal Time and Greenwich Mean Time.

7:17 AM Friday, Coordinated Universal Time (UTC) is
7:17 AM Friday, Greenwich Mean Time (GMT)

Key difference: Both UTC and GMT are time standards that differ in terms of their derivation and their use.

To quote timeanddate.com:

The Difference Between GMT and UTC:

Greenwich Mean Time (GMT) is often interchanged or confused with Coordinated Universal Time (UTC). But GMT is a time zone and UTC is a time standard.

Although GMT and UTC share the same current time in practice, there is a basic difference between the two:

• GMT is a time zone officially used in some European and African countries. The time can be displayed using both the 24-hour format (0 - 24) or the 12-hour format (1 - 12 am/pm).
• UTC is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. This means that no country or territory officially uses UTC as a local time.

Solution 2

Astronomy versus Atomic clock

By the original definitions the difference is that GMT (also officially known as Universal Time (UT), which may be confusing) is based on astronomical observations while UTC is based on atomic clocks. Later GMT has become to be used at least unofficially to refer to UTC, which blurs the distinction somewhat.

GMT stands for Greenwich Mean Time, the mean solar time at the Royal Observatory in Greenwich on the south bank in Eastern London, UK. When the sun is at its highest point exactly above Greenwich, it is 12 noon GMT. Except: The Earth spins slightly unevenly, so 12 noon is defined as the annual average, mean of when the sun is at its highest, its culmination. In GMT there can never be any leap seconds because Earth’s rotation doesn’t leap.

UTC, which stands for Coordinated Universal Time in English, is defined by atomic clocks, but is otherwise the same. In UTC a second always has the same length. Leap seconds are inserted in UTC to keep UTC and GMT from drifting apart. By contrast, in GMT the seconds are stretched as necessary, so in principle they don’t always have the same length.

For roughly 100 years GMT was used as the basis for defining time around the world. Since the world these days mostly bases precise definition of time on atomic clocks, it has become customary to base the definition of time on UTC instead.

Edit: The original meaning of GMT is somewhat useless these days, but the three letter combination doesn’t seem to go away. I take it that it is often used without regard to whether UTC is really intended, so don’t put too much trust into the strict definition given above.

For your questions:

1. Yes, time can be captured in UTC alone. Storing time in UTC and using UTC for transmitting date-time information is generally considered good practice.
2. I suppose it’s up to each state of the US to define its time. And I don’t know, but I suppose that today they (officially or in practice) define time as an offset from UTC rather than GMT. The difference between the two will always be less than a second, so for many purposes you will not need to care. Central Standard Time (for example America/Chicago) is at offset -6, as is Mountain Daylight Time (for example America/Denver). On the other hand, offset -6 doesn’t necessarily imply a time in the US. Parts of Canada and Mexico use it too, of course, and also Galapagos and Easter Island.
3. I don’t think you got your example time exactly right, but yes, 2 January 2018 at 00:00 UTC is the same point in time as 1 January 2018 at 18:00 in Chicago and other places that are at UTC-6 in winter (winter on the Northern hemisphere, that is).

Further reading:

• Systems of Time.
• Current Millis with a simple and a complex take on UTC vs. GMT.

Solution 3

❌ The accepted Answer is neither correct nor useful.

✅ In contrast, the Answer by Ole V.V. correctly summarizes the technical differences — for details follow the links to detailed pages in Wikipedia.

For programmers building business-oriented apps, the upshot is that UTC is the new GMT. You can use the terms interchangeably, with the difference being literally less than a second. So for all practical purposes in most apps, no difference at all.

Here is some more practical advice, with code examples.

Strings

Say, I have UTC time as "02-01-2018 00:03" does that mean my US local time is "01-01-2018 18:00"?

That first part is a bad example, with the date-time string lacking an indicator of its offset or zone.

If a string indicates a specific moment, it must indicate either a time zone (Continent/Region formatted name) and/or an offset-from-UTC as a number of hours-minutes-seconds. If the string is meant to represent a moment at UTC itself, that means an offset-from-UTC of zero.

To write that string with an offset, various conventions may be applied. The best in practice is with both hours and minutes along with a colon, such as +00:00, +05:30, or -08:00. The leading zero and the colon are both optional but I have seen libraries break when encountering a value such as -0800 or -8.

Zulu

As a shortcut for an offset of zero, the letter Z is commonly used to mean UTC itself. Pronounced Zulu.

ISO 8601

Furthermore, best practice in formatting date-time textually for computing is to us the ISO 8601 standard formats. For a date-time the format YYYY-MM-DDTHH:MM:SS±HH:MM:SS is used. The T separates the date portion from the time-of-day portion. This format has advantages such as being largely unambiguous, easy to parse by machine, easy to read by humans across cultures. Another advantage is sorting alphabetically is also chronological. The standard accepts the Z abbreviation as well.

So your example UTC time as "02-01-2018 00:03" is better stated as 2018-01-02T00:03Z.

java.time

Be very aware that most programming languages, libraries, and databases have very poor support for date-time handling, usually based on a poor understanding of date-time issues. Handling date-time is surprisingly complicated and tricky to master.

The only decent library I have encountered is the java.time classes (see Tutorial) bundled with Java 8 and later, and its predecessor the Joda-Time project (also loosely ported from Java to .Net in the Noda Time project).

In java.time, a moment is represent in three ways. All have a resolution of nanoseconds.

• Instant
  Always in UTC. Technically, a count of nanoseconds since the epoch reference of first moment of 1970 (1970-01-01T00:00:00Z).
• OffsetDateTime
  A date with time-of-day in the context of a certain number of hours-minutes-seconds ahead of, or behind, UTC.
• ZonedDateTime
  A date with time-of-day in the context of a certain time zone.

So what is the difference between a time zone and an offset-from-UTC? Why do we need separate classes? An offset-from-UTC is simply a number of hours-minutes-seconds, three numbers, no more, no less. A time zone in much more. A time zone is a history of the past, present, and future changes to the offset used by the people of a particular region.

What changes? Changes dictated by the whims or wisdom of their politicians. Politicians around the world have shown a predilection for changing the offset used by the time zone(s) in their jurisdiction. Daylight Saving Time (DST) is one common pattern of changes, with its schedule often changed and the decision to enact or revert from DST sometimes changed. Other changes happen too, such as just in the last few years North Korea changing their clock by half-an-hour to sync with South Korea, Venezuela turning back their clock half-an-hour only to jump back forward less than a decade later, Turkey this year canceled the scheduled change from DST to standard time with little forewarning, and contemporary Russia having made multiple such changes in recent years.

Back to your example in your point # 3, let's look at some code.

Say, I have UTC time as "02-01-2018 00:03" does that mean my US local time is "01-01-2018 18:00"?

Your example strings have another problem. That 03 minute in the first part is ignored your second part, an apparent typo. I know because there is no time zone adjustment in effect in the Americas on that date involving a fractional hour of 57 minutes.

Not a moment

First, we parse your input string. Lacking any indicator of zone or offset, we must parse using the LocalDateTime. The name LocalDateTime may be misleading, as it does mean a specific locality. It means any or all localities. For more explanation, see What's the difference between Instant and LocalDateTime?.

String input = "2018-01-02T00:03" ;                  // Text of a date with time-of-day but without any context of time zore or offset-from-UTC. *Not* a moment, *not* a point on the timeline.
LocalDateTime ldt = LocalDateTime.parse( input ) ;   // Parsing the input as a `LocalDateTime`, a class representing a date with time but no zone/offset. Again, this does *not* represent a moment, is *not* a point on the timeline. 

UTC

By the facts given in the Question, we know this date and time was intended to represent a moment in UTC. So we can assign the context of an offset-from-UTC of zero hours-minutes-seconds for UTC itself. We apply a ZoneOffset constant UTC to get a OffsetDateTime object.

OffsetDateTime odt = ldt.atOffset( ZoneOffset.UTC );    // We are certain this text was intended to represent a moment in UTC. So correct the faulty text input by assigning the context of an offset of zero, for UTC itself.

Time zone

The Question asks to see this moment through a wall-clock time of six hours behind UTC used in the United States. One time zone with such an offset is America/Chicago.

Specify a proper time zone name in the format of continent/region, such as America/Montreal, Africa/Casablanca, or Pacific/Auckland. Never use the 2-4 letter abbreviation such as CST, EST, or IST as they are not true time zones, not standardized, and not even unique(!).

ZoneId z = ZoneId.of( "America/Chicago" ) ; // Adjust from UTC to a time zone where the wall-clock time is six hours behind UTC.
ZonedDateTime zdt = odt.atZoneSameInstant( z ) ;

See this code run live at IdeOne.com.

odt.toString(): 2018-01-02T00:03Z

zdt.toString(): 2018-01-01T18:03-06:00[America/Chicago]

Same moment, different wall-clock time

This odt and zdt both represent the same simultaneous moment, the same point on the timeline. The only difference is the wall-clock time.

Let's work an example, using Iceland where their time zone uses an offset-from-UTC of zero hours-minutes-seconds. So the zone Atlantic/Reykjavik has a wall-clock time identical to UTC. At least currently today their wall-clock time matches UTC; in the past or future it may be different, which is why it is incorrect to say “UTC is the time zone of Iceland”. Anyway, our example… say someone in Reykjavík, Iceland with 3 minutes after midnight on the clock hanging on their wall makes a phone call to someone in the US. That US person lives in a place using the Chicago region time zone. As the person called picks up their phone, they glance up at the clock hanging on their wall to see the time is just after 6 PM (18:03). Same moment, different wall-clock time.

Also, the calendars hanging on their walls are different, as it is “tomorrow” in Iceland but “yesterday” in mainland US. Same moment, different dates!

---

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About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8, Java SE 9, Java SE 10, Java SE 11, and later - Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Most of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Solution 4

GMT is a mean solar time calculated at the Greenwich meridian. https://www.rmg.co.uk/discover/explore/greenwich-mean-time-gmt

UTC is based on the extremely regular "ticking" of caesium atomic clocks. https://en.wikipedia.org/wiki/Coordinated_Universal_Time

They are neither based on the same time nor calculated the same way. IMHO, the wording on https://currentmillis.com is misleading, at best, if not just flat out incorrect.

View more solutions

Comments

[Anagha]

I have seem similar questions which does not include coding. You can even suggest if anyway thru coding i can get a clear picture rather than voting it to be closed. Thank you!

[Ole V.V.]

Though conceptually different, I have never seen a computer program where it wasn’t OK to regard GMT and UTC as equivalent. They are never more than 1 second off from each other, and computer software very often does not distinguish.

[Matt Johnson-Pint]

"US local time" is not a thing. The United States has several different time zones.

[Ole V.V.]

This question is very relevant to programming. I certainly believe it belongs here.

[Paul M]

Another factoid regarding time. Blockchain has poor timekeeping features, it don't matter that much, one or two seconds between friends is/are not a good reason to have an argument.

[Anagha]

Thank you, so to get it clear, is there any time that can be captured just as UTC and not UTC-6?

[Ole V.V.]

The only Europaen country in GMT is Iceland. You are right that there are some African countries too. Curiously also the weather station Danmarkshavn in Greenland uses GMT.

[Basil Bourque]

Incorrect. GMT is not a “time zone”. GMT is the old-school marker for defining offsets around the world. We must pick some arbitrary spot on earth so as to say all other spots are a certain number of hours-minutes-seconds ahead or behind that. In modern history, that arbitrary spot became the Royal Observatory in Greenwich, London, England, UK, the G in GMT. GMT is not a time zone, it is the measure by which all other time zones are measured. A time zone is the history of past, present, and future changes to the offset-from-GMT (now UTC) used by the people of a particular region.

[user2031839]

GMT and UTC are time zones too. You can check tz database to ensure: en.wikipedia.org/wiki/List_of_tz_database_time_zones

[GrixM]

There IS a difference between UTC and GMT, because they are defined differently. The difference is kept under a second using leap seconds but that can still be critical in some cases so this answer is bad.

[pixelstuermer]

Thanks a lot. IMHO this is one of the best comments on the issues of date-time-related programming.

[MichaelChirico]

Minor note that has indeed caused major headaches nonetheless if ignored "sorting alphabetically is also chronological" is only true if subsecond times are ignored. In general I've found things like SS.123Z and SSZ (i.e. SS.000Z) both appear and this ruins string sorting.

[JohnChris]

This is why i downvoted: This answer is wrong please read here: currentmillis. UTC and GMT are currenlty derived in the same way. Historically you are correct but no longer the case

[JohnChris]

if you read further you will see that true GMT is no longer the same. It used to be calculated like above, but UTC has forced it to change and now GMT is the same as UTC. Don't delete the link, it works sorry (i edited my comment). You can check multiple sources that confirm my comment, I can send you more links if you want - I hope you change your answer to clarify <3

[Ole V.V.]

Thanks, @JohnChris, for trying to explain the downvote. Form your link: ”Unlike GMT which is based on solar time and originally calculated a second as a fraction of the time it takes for the Earth to make a full rotation around its axis, UTC calculates a second as “the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom”.” Isn’t that exactly what I habe also said, only with different words? I will delete the broken link, thank you.

[JohnChris]

The accepted answer is correct. GMT and UTC are based on the same time, calculated the same way, you are looking at how historically GMT was calculated. GMT is a timezone though and thats the only difference, which the accepted answer specifies. Please fix your answer as to not spread misinformation

[JohnChris]

@GrixM you are wrong, historically you are correct, but with a bit of research you will find that the accepted answer is correct. check here

[JohnChris]

@BasilBourque you are wrong, historically you are correct, but with a bit of research you will find that the accepted answer is correct. check here

[Mark Douglas]

@BasilBourque So what time zone does the UK use if GMT is 'not a time zone'?

[Basil Bourque]

@MarkDouglas The mainland of the U.K. uses the time zone Europe/London. This zone currently alternates between around half the year at zero hours-minutes-seconds from UTC (GMT) +00:00 and half the year at one hour ahead of UTC (GMT) +01:00. Other parts of the U.K. use other zones with various offsets.

[Basil Bourque]

@JohnChris The offset of a time zone can change at the whim of politicians. GMT never changes. A locality may choose to use an offset of zero hours-minutes-seconds from UTC/GMT as the offset of their own time zone. Later they may choose to observe Daylight Saving Time (DST). At that point in time, half the year their offset-from-UTC is one hour ahead of UTC/GMT. That locality’s decision does not change GMT, it changes their time zone. As I said in the Answer, a time zone is a history of the past, present, and future changes to the offset used by the people of a particular region.

[Himanshu Saini]

Plus one for being straight to the point in first statement

[JustinP]

@JohnChris The link you cite in your comments below, currentmillis.com says literally ' If you were to calculate true GMT today ... this would for sure return a different absolute value than what UTC gives us". How does this support your contention they are the same?

[Basil Bourque]

@JustinP Your quotation skipped over the "original definition of 1 second = 1/86400 days" part. We are talking about a fraction of a second in difference. When the drift in difference accumulates to a full second over a few years, the scientific body in charge declares a Leap Second. If you are doing common business-oriented programming, this difference is obviously irrelevant. If you are working with GPS/Galileo satellite signaling or literally rocket science, then yes there is a difference.

[pabouk - Ukraine stay strong]

As I understand it: Currently GMT is most often used as the name of the UTC+00 time zone at the British Isles. Before that GMT was what UT is now - universal time based on Earth's rotation measured by astronomical observations.

[Ole V.V.]

@pabouk-Ukrainestaystrong Thanks for summarizing. Yes, that’s a use of GMT that I clearly recognize. I think it’s used a bit broader than that. Otherwise our understandings agree.