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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

by chaz6 5y ago

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.

deleted 5y ago
[deleted]
jonny_eh 5y ago
> UTC is based on TAI What is TAI?
- mburns 5y ago
  https://www.fhs.swiss/eng/gmt-utc-tai.html https://www.fhs.swiss/eng/gmt-utc-tai.html
- LegionMammal978 5y ago
  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.
kemitche 5y ago
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-time-frequently-asked-questions-faq https://www.nist.gov/pml/time-and-frequency-division/nist-ti...
- HatchedLake721 5y ago
  Thank you. I really don’t understand people who use rare acronyms on a global forum and don’t write what it is.
- joshxyz 5y ago
  Thank you for the link
sillyquiet 5y ago
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?
- afavour 5y ago
  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.
- gmiller123456 5y ago
  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.
rattray 5y ago
Will the two ever provide different values?
Groxx 5y ago
If UTC is corrected to match GMT: what's the difference, aside from conceptual?
- postalrat 5y ago
  My understanding is it's corrected with whole leap seconds. So they could differ by a fraction of a second.
- below43 5y ago
  GMT is different from UTC during British Summer Time (UTC+01:00)
  - Flimm 5y ago
    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.
gmiller123456 5y ago
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/EarthOrientationData/eop.html https://www.iers.org/IERS/EN/DataProducts/EarthOrientationDa... See "finals.all IAU2000"
fanf2 5y ago
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.

by chaz6 5y ago

deleted 5y ago

[deleted]

jonny_eh 5y ago

> UTC is based on TAI What is TAI?

mburns 5y ago
https://www.fhs.swiss/eng/gmt-utc-tai.html https://www.fhs.swiss/eng/gmt-utc-tai.html
LegionMammal978 5y ago
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.

kemitche 5y ago

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-time-frequently-asked-questions-faq https://www.nist.gov/pml/time-and-frequency-division/nist-ti...

HatchedLake721 5y ago
Thank you. I really don’t understand people who use rare acronyms on a global forum and don’t write what it is.
joshxyz 5y ago
Thank you for the link

sillyquiet 5y ago

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?

afavour 5y ago
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.
gmiller123456 5y ago
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.

rattray 5y ago

Will the two ever provide different values?

Groxx 5y ago

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

postalrat 5y ago
My understanding is it's corrected with whole leap seconds. So they could differ by a fraction of a second.
below43 5y ago
GMT is different from UTC during British Summer Time (UTC+01:00)
- Flimm 5y ago
  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.

gmiller123456 5y ago

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/EarthOrientationData/eop.html https://www.iers.org/IERS/EN/DataProducts/EarthOrientationDa... See "finals.all IAU2000"

fanf2 5y ago

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.