[LEAPSECS] UTC Redefinition Advanced

Jonathan E. Hardis jhardis at tcs.wap.org
Sat Oct 23 15:00:25 EDT 2010

On Oct 23, 2010, at 1:48 PM, Hal Murray wrote:


>> How many of these systems CURRENTLY properly handle leap seconds?

>> How

>> many of these cell phones and space systems and digital devices

>> "buried

>> beneath Antarctic ice" CURRENTLY are built to a specification that a

>> minute can contain either 59, 60, or 61 seconds? Or that when a leap

>> second occurs, it occurs at midnight only in the UTC+0 time zone?

>> (4:00

>> PM in the afternoon in California)


> Why is that such a big deal? At worst, they reboot the system and

> it gets

> back in sync with the loss of some data.

Okay. At 4:00 PM PST on December 31 -- one hour before the close of
business on the last business day of the year -- you would have folk
on the West Coast reboot all of their financial systems, their
communications systems, their "smart grid" electrical systems, and
whatever else happens to sync to UTC.

Yeah, I know that we've had a lot of leap seconds over the years, and
people have dealt with them one way or another -- mostly
improvisationally, not following standards. But it's becoming tougher
and tougher to deal with.

To reiterate, my point is that people who think leap seconds are a
good idea (and that includes me, by the way), seem to be complaining
at the wrong end of the value chain. Having leap seconds is
counterproductive if they choke the infrastructure. Take a "time
out," on declaring leap seconds, direct the frustration towards fixing
the infrastructure, and then some years from now have the discussion
again on whether leap seconds would be a good idea or not.

> I think the more important issue would be calculating differences in

> times

> that straddle a leap second. (I'm not an astronomer.)

Here's the issue. There's a number called "DUT1," which is the
difference (in seconds) between UTC, an atomic time scale, and UT1, a
solar-based time scale. Right now, DUT1 is constrained by agreement
to be 0.9 s, or less, in magnitude (+/-). The proposal is to allow
DUT1 to grow larger than 0.9 s, and that would affect systems (the few
that care about it, mostly astronomical) that cannot handle these
larger numbers.

Once you get over the hurdle of allowing DUT1 to be larger than 0.9,
many additional possibilities open up. You could concentrate the pain
by having a "leap minute" once a century rather than a "leap second"
every year or two. Alternatively, you could declare leap seconds 50
or 100 years in advance, which presumably would make their
implementation easier and more straightforward. Or, society could
decide that having a leap-anything isn't worth the bother.

- Jonathan

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