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Comment by chaz6 | Hacker News Reader

It is disappointing to see GMT conflated with UTC. It is not "UTC plus zero hours". GMT is based on solar time, wheras UTC is based on TAI. UTC is TAI corrected (with leap seconds) to match GMT. Hopefully I have got that right.

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GMT is not (officially) used anymore. UT1 is based on the Earth's rotation within the ICRF (almost J2000) reference frame based on the positions of distant quasars. UT1 is not a steady time frame, e.g. if you measure UT1 at two times one second apart, it probably will differ by something other than one second. This is due to the fact that the Earth's rotation rate is constantly changing.

UTC is what people normally mean when they refer to GMT or Universal Time. It has a steady definition for a second based on TAI, and is adjusted by adding leap seconds to keep it within .9 seconds of UT1.

Every day the International Earth Rotation Service publishes values for UT1-UTC based on actual observations as well as other Earth Orientation Parameters. They are also responsible for determining and announcing in advance of when a leap second will be added.

E.g.: https://www.iers.org/IERS/EN/DataProducts/EarthOrientationDa... See "finals.all IAU2000"

While all that is super cool and good to know, does it change or affect at all how js dev or their app interacts with date or time since they are for all practical purposes synonymous?

One huge difference between them is that UTC is, well, universally used, while GMT is switched out for BST (an hour ahead) in summer months on client machines. Not that you should be assuming anything about the state of the client machine but this is an extra footgun.

Probably not, but there's no reason to perpetuate the confusion of the two. I'd think the only practical situation where it'd be a problem is if you enter GMT into software that actually interprets it as GMT rather than UTC.

For any readers who, like me, weren't sure of the differences (or what "TAI" refers to), this page helped clear things up for me: https://www.nist.gov/pml/time-and-frequency-division/nist-ti...

Thank you. I really don’t understand people who use rare acronyms on a global forum and don’t write what it is.

Thank you for the link

I am afraid that’s wrong. What you are calling GMT is actually called UT1.

GMT does not exist as a separate thing any more. It was the timescale maintained by the Greenwich observatory, but their timescale matched UTC since the rubber seconds era in the 1960s, and in 1972 Greenwich time became UTC with leap seconds. The Greenwich observatory as an institution does not exist any more (there’s a museum where it used to be but it isn’t a working national scientific laboratory). GMT is now an informal synonym for UTC that still appears in things like British law for historical reasons.

Will the two ever provide different values?

> UTC is based on TAI

What is TAI?

https://www.fhs.swiss/eng/gmt-utc-tai.html

TAI is International Atomic Time. It is used as the reference point for UTC, which is defined as TAI minus a fixed number of seconds (currently 37). To add or remove a leap second, the TAI - UTC offset is adjusted. Leap seconds have no effect on TAI itself, which always runs at 1 second per SI second, as measured by a consensus of atomic clocks.

If UTC is corrected to match GMT: what's the difference, aside from conceptual?

My understanding is it's corrected with whole leap seconds. So they could differ by a fraction of a second.

GMT is different from UTC during British Summer Time (UTC+01:00)

That's incorrect. GMT is always the same as UTC for all intents and purposes (setting aside things like leap seconds). British Summer Time is GMT+01 (which is the same as UTC+01). The time zone in England is Europe/London, which uses GMT in the winter, and GMT+01 in the summer.

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