Re: [LEAPSECS] Risks of change to UTC

From: M. Warner Losh <imp_at_BSDIMP.COM>
Date: Sat, 21 Jan 2006 15:15:32 -0700 (MST)

In message: <>
            "Daniel R. Tobias" <> writes:
: On 21 Jan 2006 at 10:11, M. Warner Losh wrote:
: > I maintain that for human activity, there's no need for leap seconds
: > at all. In each person's lifetime, the accumulated error is on the
: > order of a few minutes. Over generations, the problems with noon
: > drifting to 1pm can trivially be solved by moving the timezones that
: > civilian time uses.
: What about when that accumulated difference is over 24 hours, so the
: offset between solar-based time and atomic time is actually on the
: order of days?

From we know that the
rate of change of the day is somewhere between 25.6 s/century^2 and 42
s/century^2. At those rates, it will be the year 6360-7633 when
enough time has accumulated for there to be a day difference. Before
then, however, we go through a number of events. Somewhere between
2058 and 2211, enter the realm where there's more than 2 leap seconds
per year. This will be the first great UTC breakage because many
devices today (ntp included) KNOW that leap seconds happen twice
yearly. The next break will happen sometime between 2300 and 2600
when we'll need more than 4 leap seconds a year. The current ITU-R
TG.460 standard for leap seconds defines only primary and secondary
leap second times. It does not define tertiary, so no one knows when
the leap seconds will happen if you need more than 4 per year, but the
ITU recommendation is that they happen at the end of a month.
Somewhere between 3250 and 4200 there will be more than 1 leap second
a month needed. At this point the ITU scheme of having the leap
second at the end of the month will need to be modified.

That's at least 2000 years before 24 hours of delta have accumulated.

For some perspective, we've been using UTC for only ~50 years and the
gregorian calendar for only ~1500 years. I'd anticipate that
something would need to be done about the slowing of the day well
before 4300 years have passed.

Somewhere around betwee 45,000-80,000 you'll need more than one leap
second a day.

: Will people be able to deal with a civil time
: standard that is based on an offset from a "UTC" that says it's
: Monday when all actual points on Earth have the local date at
: Saturday or Sunday?

Since that's 4k or more years into the future, who alive today will
know enough about what the future is like to impose a scheme that is
guranteed to work?

Clearly some scheme better than leap seconds will need to be invented
well in advance of these events. A new scheme will be needed well in
advance of the Tuesday is really Wednesday problem.

: Many Web sites (including Wikipedia) use UTC as
: the standard for date/timestamps; will this be a reasonable thing
: when this causes the date of postings to be far off from what is
: being used locally? And when, at some future point, the Gregorian
: calendar itself needs adjustment to handle the fact that it doesn't
: get the length of the year precisely correctly (and the length of the
: year in terms of solar days is changing due to the lengthening of the
: day, anyway), will this adjustment be done to the UTC standard (why,
: when it doesn't follow astronomy anyway?), or as an additional offset
: to local times (which could result in different countries having
: different dates as in the Julian/Gregorian transition period)?

The length of the gregorian calendar is off by 23s per year. In year
5500 or so we'll have accumulated a day of error in it and we'll need to
skip a leap year to correct for that problem. This is a good 2000
years before we'll have accumulated a day of DUT.

As you can see from the above, leap seconds won't save you. They will
run out of steap in about 1500 to 2500 years. At that point the
accumulated difference will be only about 2 hours. If leap seconds
are totally abolished, time zone transitions could easily continue for
about 4000 years. Either way, you have a problem. The length of the
SI second is fixed, and the length of the day is getting shorter.

1500 years ago, no one spoke English. Chances are the people that
deal with this problem in 1000 or 2000 years won't speak any language
recognizable to anybody alive today.

Received on Sat Jan 21 2006 - 14:16:13 PST

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