GMT Versus UTC - Time And Date

When you want to express the time in a time zone, do you use UTC or GMT as a reference point?

It may seem like a moot question—after all, both share the same current time. So, in practice, saying that New York is 5 hours behind UTC is the same as saying that it’s 5 hours behind GMT.

However, there are two good reasons to use UTC instead of GMT.

UTC vs GMT: What’s the Difference?

Let’s start by defining the main difference between the two:

• UTC is a time standard officially used to determine the civil time in time zones worldwide.
• GMT is a time zone observed in some European and African countries.

Beyond Local Time

UTC is also used as an overarching, global time, especially when local times and time differences are irrelevant and may cause confusion or unnecessary noise.

For example, space agencies may use it to time their mission; similarly, some Antarctic bases operate on UTC; militaries typically also use it during global missions, albeit usually under the name Zulu Time.

UTC: the World’s Time Standard

So, the first reason to use UTC (Coordinated Universal Time) is that it is the correct, official choice. The world’s governments and regulatory bodies have agreed to use it as the sole global reference point for time zones.

When a country determines its time zone, the law text will show the difference, or offset, of the local time from UTC. For example, Japan has decided to be nine hours ahead of UTC, so the country’s time zone, Japan Standard Time (JST), is officially defined by its offset of UTC+9.

UTC offsets of all time zones

GMT: Just Another Time Zone

Meanwhile, GMT (Greenwich Mean Time) is a time zone, not a time standard. Using it as a reference point for other time zones is technically incorrect.

Like all other time zones, its current time is derived from UTC—it just so happens that the UTC offset is zero. So, GMT equals UTC, and a GMT clock ticks in perfect unison with a UTC clock.

Always Use 24-Hour Clock for UTC

Speaking of doing things correctly: Another quirk of UTC time is that you must use the 24-hour notation. For example, 7 o’clock in the evening must be written as 19:00 UTC. It is incorrect to write 7:00 pm UTC.

Conversely, GMT can be written in both the 24-hour and the 12-hour format (with am and pm).

Learn the 24-hour clock

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Greenwich Mean Time Doesn’t Equal Time in Greenwich

The second—and perhaps more important—reason not to use GMT when communicating time differences is that it can lead to confusion.

Greenwich Mean Time is one of the most well-known time zones worldwide. After all, it used to be the world’s time standard before UTC came along. Most people will know that it was initially based on the mean solar time at the prime meridian that runs through the Royal Observatory in Greenwich, England—hence its name. By extension, people tend to associate GMT with the time in England.

However, while the United Kingdom observes GMT as standard time, it doesn’t do so all year: Like most other European countries, the country uses Daylight Saving Time (DST). Every March, Brits switch to British Summer Time (BST), making their clocks run one hour ahead of GMT (UTC+1).

In our experience, this is where people often go wrong: equating GMT with the time in the UK. Quite regularly, our support team responds to messages about missed meetings or supposed mistakes in our Time Zone Converter or Meeting Planner—all because of this false assumption.

So, to be safe, we recommend you use UTC when communicating about time zones and time differences.

Why it’s so hard to convert time zones in your head

UTC, GMT, and Daylight Saving Time

Speaking of clock changes, it is worth noting that neither UTC nor GMT ever changes for DST.

They keep ticking away at the same synchronous pace, even though countries like the UK, which use GMT as standard time, switch to different time zones when DST begins.

By the way, that’s why you should always select actual locations when using our Time Zone Converter or Meeting Planner instead of entering UTC, GMT, or any other time zone. That way, all clock changes will be automatically taken into account.

Topics: Timekeeping, Time Zone