Leap second tonight

Mon Jan 5 22:04:51 UTC 2009

>>> On 1/5/2009 at 1:19 PM, Peter Beckman <beckman at angryox.com> wrote:
> I've gleened from this thread that:
> 
>      * everyone uses UTC, or should, because UTC is a uniform time scale,
>        except for those leap seconds

Local time is totally appropriate in some circumstances, but it
is pretty much always defined as just an offset to UTC. UT1 is
important when you are doing astronomical observations or depend
on such things.

>      * UTC is sourced from the frequence of a radio emission from cesium
>        atoms which are extremely constant

There is nothing too special about cesium. It's properties and
the properties of the particular transition are convenient from
an engineering standpoint.

>      * UTC can get out of whack with the rotation of the earth around the
>        sun, because our rotation is not uniform, but is calculated rather
>        than measured (well, sort of)

No its not about years, that's what February 29th is for, it's about
days. As the Earth rotates (there's a "rotation" versus "revolution"
nomenclature), the sun appears to move around it daily. This is the
solar day. At a certain point, at a certain location, the sun is
at the highest it will be in the sky for that rotation. This is solar
noon. The time between two consecutive noons at any location is not
constant mostly due to the Earth's orbit being elliptical and the tilt
of the Earth's axis. But for any place on Earth, the mean solar day,
the average of the solar day over the year, is the same.

If the Earth was a solid sphere rotating at a constant speed, that
would be the end of the story. But it's not and for a variety of
reasons, the rotation of the Earth changes with time (mostly slows).
This causes the mean solar day to get longer.

>      * UTC is TAI plus leap seconds.  In 1972, when leap seconds were first
>        introduced, UTC was TAI - 10 seconds.  UTC is now TAI - 34 seconds.
>        TAI ticks exactly as fast as UTC, ignoring leap second adjustments.
>      * UTC is slower than UT1 by about 1ms per day.

That's a very tricky sentence. I think that the mean solar day right
now is about 86400.002 s long. The mean solar day was actually exactly
86400 s long sometime around 1820.

>      * On 12/31/2008 UTC was (-) 591.8ms behind UT1.  On 1/1/2008 UTC was
>        407.1ms ahead of UT1.
>      * Leap seconds are applied to UTC every few years to remain in line
>        with UT1, the time based on the rotation of the earth around the sun.

A time based on the rotation of the Earth.(period) As mentioned
above, it doesn't really have anything directly to do with Earth's
location in its orbit.

>      * GMT is used to imply UT1, but sometimes UTC, but really GMT is just
>        massively confusing and you shouldn't use it, either in conversation
>        or in your servers/routers, because nobody is really sure without
>        reading a lot of documentation what GMT means for each
>        manufacturer/OS/software.
>      * When writing code regarding dates and times, know that any year may
>        have 366 days, and any minute may have 61 seconds.

Also remember that an administrator or automated clock recalibration
may take you forward or back in time any arbitrary amount.

>      * When in doubt, Dr.  Daniel Gambis is always right.

Very, very few should not defer to his judgement on such matters.