[time-nuts] Time offset

Magnus Danielson magnus at rubidium.dyndns.org
Thu Mar 19 09:29:07 UTC 2009

M. Warner Losh skrev:
> 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.

Warner has understood my quest here...

> 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.

It would be an interesting development if countries left UTC for say UT2 
as their official time if leap seconds would cease. It would effectively 
make the change of leap second policy cause UTC a less and less valuable 
source. I am becoming more and more sceptical to the wisdom of dropping 
the leap seconds or at least leap mechanism.

> : 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.

It may be OK with current UTC and it may not. UTC may cease to be a 
reasonable interpretation if leap seconds ceases.

> : 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...

The nitty gritty details...


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