Re: [LEAPSECS] Unifying Atomic Time and the post-Gregorian calendar corrections

From: Markus Kuhn <>
Date: Thu, 03 Jul 2003 18:30:00 +0100

Steve Allen wrote on 2003-07-03 16:15 UTC:
> Okay, but in the Caves of Steel where people use TI-based timekeeping
> 5600-03-01 is a Wednesday, and in the UT-based lands where the sun
> determines when people work and sleep 5600-03-01 is a Tuesday.

Thanks for pointing this out! Weekday-continuity is indeed a bit of a
problem of this approach. I can't see any other practical solution than
skipping one weekday together with the skipped 29 February 5600, in
other words, 5600-03-01 would be a Wednesday also in all the local
civilian time zones.

Otherwise, computers would not be able to calculate the day of the week
from a YYYY-MM-DD date without an extra table that keeps track of all
the rare (every 3000 years or so) exceptions to the Gregorian rule
triggered by British Time < TI - 11h overflow. Any need for software
functions that are tested only every 3000 years can safely be considered
bad practice.

I understand that none of the past calendar reforms did interrupt the
mod-7 weekday cycle. And references such as

argue even (through an absence of documented evidence to the contrary)
that the 7-day week might have been running uninterrupted for at least
the past 3000 years.

But then, they didn't have computer software to worry about during any
of the past calendar reforms. I think user requirements might have
changed for weekdays.

The Unix command "cal 03 5600" tells me today that 5600-03-01 will be a
Wednesday. It is also a form of long-term continuity that we stick to
that algorithm, no matter whether the Gregorian leap day 5600-02-29
actually took place or not.

> I am not convinced that there is anyone who does long-term solar
> system stability studies who would be willing and able to comment on
> this in an authoritative fashion. It's just too far into the future.

Fair enough. Though it would be academically interesting to see some
discussion of what the contributing factors are for changes in the
length of the tropical year. For length-of-day changes, we have have
been told about tidal breaking and other effects. Are there similar
theories for the long-term change of the Earth's orbit (change of solar
mass, solar wind friction, etc.)?

According to

astronomers such as John Herschel (1792-1871) and others suggested in
the past to augment the Gregorian calendar with a rule that makes every
year divisible by 4000 not a leap year. Similarly, the Orthodox church
in Greece considered in the 1920s to replace the Gregorian "Y mod 400 =
0" rule with a "Y mod 900 in {200, 600}" rule. I'd be curious to hear,
whether the usefulness of such proposals have ever been discussed more
recently in the light of modern astrometric and orbital-modeling
capabilities. Or do we first have to build a vast solar-system wide
equivalent of GPS to gather data about the solar gravitation and
pressure environment with sufficient accuracy for long-term forecasting?

Surely a government that feeds both a major space exploration agency and
a significant number of political representatives with close ties to
religious fundamentalism must be eager to fund research on when exactly
Easter will happen in the year 100000 anno domini ...


Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__
Received on Thu Jul 03 2003 - 10:30:14 PDT

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