“I’m confused. In Outlook there are two choices for a UK/London time zone. UTC or London. Both are zero meridian (+00:00). What’s the difference and why isn’t an entry for GMT?” Oscar R, Missouri.
The quick answer is to always choose the time zone entry that matches the city or country (near enough). Ignore the ‘pure’ UTC options unless you have a specific need.
The time zone selectors in Outlook and Windows let you choose not just an offset from UTC time but also any daylight saving/summer time changes.
The entry for ‘Dublin, Edinburgh, Lisbon, London” isn’t just for that time zone but also to allow the ‘spring forward/leap back’. Windows gets information on the dates each country changes on/off summer time which Outlook uses to show appointments at the right time.
UTC is a globally agreed time which never adjusts for daylight savings.
Choose the city/country time zone entry, not the UTC one. That will make sure your appointments are correctly adjusted for ‘summer time’ and not an hour out.
(Nerd note: Outlook saves all appointments as UTC time plus the time zone setting. Regular users don’t have to worry about that because Outlook handles the adjustment to ‘local’ time automatically.
When to use UTC
Perhaps the most common user of UTC time are airlines and aviation. Airline scheduling is done in UTC which is converted to local time when we passengers see it.
Web connected servers synchronize time between each other using UTC.
Other companies and organizations will use UTC to coordinate their work across the globe. Individual appointments can be created with the time zone set for UTC.
UTC vs GMT vs Zulu
Where does GMT fit into all this? Greenwich Mean Time (GMT) is almost the same as UTC. The two may differ by up to a second.
Greenwich Mean Time has been around for several centuries, being used by seafarers to calculate longitude.
In 1960 a global time standard was introduced. The Coordinated Universal Time standard was agreed with the London meridian as the zero point (there was a push to make it Paris instead).
Why isn’t the acronym CUT? Part of the global agreement is that ‘UTC’ would be the acronym regardless of language. It was a compromise between the English and French.
UTC is adjusted to match tiny changes in the Earth’s rotation. You may remember articles about ‘losing a second’ or less at the end of a year. That’s important for synchronizing servers and scientific equipment.
GMT doesn’t get those tiny adjustments so it’s very slightly different from UTC, but only up to 0.9s difference.
In many places, especially in Britain, the terms UTC and GMT are used interchangeably.
The military and aviation talks about a time ‘Zulu’ which sounds impressive, especially in TV shows.
But it’s just UTC by another name using the NATO phonetic alphabet.
If you need to set an Outlook appointment for UTC / GMT or Zulu time, use the ‘UTC’ option in Outlook’s time zone list.
Otherwise, choose the ‘London’ option for everyday use for Great Britain and Ireland.