Warner Losh imp at bsdimp.com
Wed Jan 4 13:25:55 EST 2012

On Jan 4, 2012, at 11:01 AM, Rob Seaman wrote:

> And yet googling "greenwich mean time" returns 5,250,000 results (353,000,000 for "gmt" but an unknown number are false positives) including many sites like:


> http://www.worldtimeserver.com/current_time_in_UTC.aspx


> that equate UTC with GMT. As well as things like:


> http://wwp.greenwichmeantime.com/


> that perpetuate the brand as being actually tied to Greenwich and Britain in general.


> Which is it? Does GMT == UTC? Or does it continue to mean what it says, mean solar time at Greenwich? It can't be both in a post-leap second world. That it was deprecated for various technical purposes (not, apparently, all) does not mean it doesn't remain widely used.

GMT can == UTC

Let me explain.

GMT is the old standard and also the old name for the basis of UT*. When UTC was created, it initially used the GMT name. However, this lead to confusion because UTC isn't UT1 or UT2, which was the old basis for GMT, so it was renamed after the fact to UTC to avoid confusion. However, this created the confusion that persists to this day.

The old usages of GMT persist. Sometimes it means exactly UTC. Usually, I'd say. But sometimes people use it to really mean UT1, especially when they talk about legal time in the UK being GMT and not UTC since the two are different. Or when they talk about having a set of NTP servers based on GMT instead of UTC based on an offset obtained from IERS.

It is also political: NASA uses GMT to mean UTC to keep the UK happy, for example, since keeping them happy was more important than keeping the French happy at some point in the past. Once that decision was made, it stayed made due to institutional momentum. There was a long thread about this a few years ago...

Since GMT's actual meaning/usage is ambiguous, the pedants prefer UTC to avoid this ambiguity.

It is the biggest example of why discontinuing leap seconds, but still calling UTC "UTC" may be dangerous from a pedagogical perspective.


> Rob

> --

> On Jan 4, 2012, at 10:49 AM, Tony Finch wrote:


>> Rob Seaman <seaman at noao.edu> wrote:


>>> It seems quite possible that the plan is similarly "to get rid of" GMT.

>>> Indeed, how can GMT continue to have a coherent meaning afterwards?


>> GMT was discontinued decades ago and has not had a coherent meaning for

>> even longer.


>> Tony.


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