[time-nuts] Time offset

M. Warner Losh imp at bsdimp.com
Thu Mar 19 01:24:44 UTC 2009

In message: <001c01c9a82e$4014b270$a101a8c0 at officemail>
            "phil" <fortime at bellsouth.net> writes:
: Magnus,
: On that same page was a link to an older archive, tzarchive.gz 
: ftp://elsie.nci.nih.gov/pub/tzarchive.gz
: You will find references to actual laws and links imbedded in that for 
: various countries. Your assumption that that GMT = UTC I would say is true 
: from 1970 on.

Except that it really isn't.  This is the whole point of Magnus'
request.  The national laws are written to specify "Mean solar time"
at a given meridian.  One realization of mean solar time is UT1, while
another is UTC.  Often things aren't specified exactly in the laws.
These two are almost interchangeable, but not quite.

For example, if UTC were redefined to omit leap seconds, the issue
could become a real one again.  The US is now on UTC time, where until
recently it was a Mean Solar Time, as defined by the Department of
Commerce, which was some variation of UT2 for a while, but quickly
became the same as UTC when the official time keeping responsibilities
transitioned to NIST.  NIST determined that UTC was a mean solar time,
and published that as the official time of the US.  With the old
definition, a change to the underlying UTC might mean the US would
have had to deviate from UTC.  With the current law, it is clear that
UTC, whatever it is, is the official time.

: GMT was the first internationally accepted international/global standard 
: with various "legally" defined offsets. It was only after the advent of the 
: cesium and the gps system that UTC became the standard, again with the legal 
: offsets. Most older law, pre 1970 I've seen references to gmt, but when laws 
: are appended for example saving time, reference is often or sometimes made 
: to utc, though the old legal definition may still reference gmt.

Right.  However, these old legal definitions that specify mean solar
time may be OK with the UTC approximation, with others may not.

: Perhaps most lawmakers accept them (gmt, utc) to be the same with their 
: local/regional offsets now that you can get standardized utc off satellites 
: world wide.

Right, but this is speculation.  Magnus is looking for the law on the
topic.  I presume both the actual law as written, but also the
regulation laws used to implement the legislative intent.

: Other than the "flying clock" how else can all countries of the world 
: synchronize their time? I think a lot of small countries have a single 
: cesium, if that, tied to gps and vend their countries "official" time based 
: on that. In that case they are based on UTC regardless of the wording of 
: their prior law.
: I know in North Carolina, USA a law was still on the books a few years ago 
: that it was illegal to look at your wife naked. Law is often slow to catch 
: up with society and technology. The various countries definitions of time 
: referencing GMT may too be laws that have not come into the twentieth 
: century though utc (with offsets) is now the accepted standard.

This is true.  I think Magnus is looking for the details...


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