[time-nuts] GMT vs. UTC
M. Warner Losh
imp at bsdimp.com
Tue Nov 21 11:29:27 EST 2006
In message: <003601c70d87$c8b37a60$7b01a8c0 at Inspiron>
"Jason Rabel" <jason at extremeoverclocking.com> writes:
: I would think each country would have a website where they declare their
: official measurements for everything (their national standards). When in
: doubt label a time with the appropriate format so people could convert it to
: whatever they want.
: Every country I would think recognizes GMT time, and specifies their time as
: certain hour offsets for local time. If you started walking around telling
: people UTC/GMT-0 time they would probably just look at you confused
: (assuming you lived in another time zone).
: >From what I gather, UTC is a precise time standard (based on Cesium) used
: for measurements and such since it is a specified constant. However, the
: fact that leap seconds are inserted in UTC time (to bring it in sync with
: GMT) means to me that GMT is the real global standard (since it's based on
: the rotation of the earth, which isn't constant).
GMT means many different things over time. Once upon a time it did
mean "whatever those silly celestial bodies, especially the sun, are
doing over that quaint little town of Greenwitch." Now it is much
closer to UTC in definition, but it can be hard to find an exact
definition because it keeps changing. The differences are < 1s (thank
you leap seconds, or curse you depending on your stripe).
Curiously, US official time is defined by statute to be something that
sounds like GMT - N hours. However, there's enough wiggle room in the
statue to allow the US to define its official time as UTC - N hours by
regulation (although I've not found the actual regulation). The
reason for this is that it is the mean solar time, as determined by
the Secretary of Commerce. The secretary delegates it to the time
labs, who have gone with UTC. These time labs participate in the
realization of TAI time, which has a "fixed" offset to UTC.
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