Greenwich, Zulu & UTC: What's the difference?

March 18, 2009 9:08 PM

What is the difference between Greenwich time, Zulu and UTC? Is Zulu and UTC a form of Greenwich time? I can't seem to find a page that states the difference. And do these forms of measurement not recognize daylight savings? Thanks in advance!
posted by cwarmy to science & nature (8 comments total)

Wikipedia to the rescue-- UTC, Zulu, and GMT are pretty much all the same, although GMT can mean either UTC or UT1 and is usually avoided as a result.

It's not subject to daylight savings, either.
posted by fairytale of los angeles at 9:12 PM on March 18

Wikipedia has the answer.

Basically, GMT is a slightly ambiguous thing; it could mean UTC or UT1 - UTC is based on an atomic clock; UT1 is based on the Sun. Zulu is UTC. None of the above are affected by daylight savings; GMT, when shifted for daylight savings, is called British Summer Time.
posted by Tomorrowful at 9:15 PM on March 18

I also thought UTC got leap seconds, but GMT doesn't. Could be apocryphal though.
posted by sbutler at 9:41 PM on March 18

That's mostly right, sbutler. Short version: the "fundamental" time standards are UT1 and TAI. UT1 is measured by the position of the sun from the Royal Observatory in Greenwich; TAI is measured by atomic clocks.

On the one hand, the earth's rotational speed isn't exactly constant, so UT1 seconds are of varying length in real, physical units of time, so UT1 is impossible to use for high-precision measurements. On the other hand, for the same reason, TAI gets out of sync with everyday notions of time as measured by the sun and stars. The solution is UTC, which is just TAI plus or minus an integer number of leap seconds, to keep it close to UT1. Greenwich Mean Time technically means UT1, but is commonly used to refer to UTC nowadays.
posted by teraflop at 10:43 PM on March 18 [2 favorites has favorites]

GMT, when shifted for daylight savings, is called British Summer Time.

GMT is always GMT. Greenwich and the rest of the UK just happens to be on BST most of the year. (GMT's still used by the BBC World Service year-round.)
posted by holgate at 10:46 PM on March 18

"Zulu Time" is the term most used in the Military. GMT can be considered to have Imperialist connotations. UTC is used when we don't want to upset our French friends :)
posted by Sturdy at 4:13 AM on March 19

It's technically "daylight saving time" (or, sometimes, "daylight-saving time"). Not "savings." But in any case, GMT/UTC/Zulu are fixed independently of DST. DST is derived by adjusting how many hours are added or subtracted from UTC in your locale. For example, in the U.S., Eastern Standard Time is defined as UTC-5 ("UTC minus five") and Eastern Daylight Time is UTC-4.
posted by Nothlit at 7:14 AM on March 19

GMT's still used by the BBC World Service year-round.

Actually, the BBC uses UTC but calls it GMT. This only matters if you think GMT = UT1 rather than GMT = UTC.

I should point out that Zulu is the NATO phonetic alphabet name for the Z used for the military timezone where the local time is UTC. The U.S. military uses other letters to refer to other timezones.
posted by grouse at 7:45 AM on March 19