[time-nuts] leap day

Warner Losh imp at bsdimp.com
Sun Nov 20 22:05:17 EST 2005

> have recently read several articles on calendars (and adjustments to
> them) of various cultures- question: i recall reading an article
> long ago about a double leap day (feb 30th) that in theory took
> place every 10k yrs- was this a concept in place prior to leap
> seconds and 'accurate' time messurement or what???

I think you are confusing two things.  There was a double leap year in
the 18th century in Sweeden.  There's also a slight error in the
current rules for a leap lear (divisible by 4, not divisible by 100
unless divisble by 400) that would be corrected if we made every year
divisible by 4000 a leap year.  Since that also doesn't produce an
exact answer, we can continue this progression, but no one has[*].  Nor
has anybody lobbied to make this modest reform to the Gregorian
calendar.  It is unclear who would have the clout that the Pope of the
sixteenth century had.  I'll note that this calendar was promulgated
in 1582, the Russian Empire (including Finland and the Baltic states)
didn't convert until the Soviet Revolutuion in 1917, only 325 years
after the fact (but to be fair, most of western europe did complete
the adaptation by 1780 or so, a mere 200 years).[*]

There once was a double leap day.  From the time zone files common in
BSD implementations we find in the Theory file:

>> In 1700, Denmark made the transition from Julian to Gregorian.  Sweden
>> decided to *start* a transition in 1700 as well, but rather than have one of
>> those unsightly calendar gaps :-), they simply decreed that the next leap
>> year after 1696 would be in 1744 -- putting the whole country on a calendar
>> different from both Julian and Gregorian for a period of 40 years.
>> However, in 1704 something went wrong and the plan was not carried through;
>> they did, after all, have a leap year that year.  And one in 1708.  In 1712
>> they gave it up and went back to Julian, putting 30 days in February that
>> year!...
>> Then in 1753, Sweden made the transition to Gregorian in the usual manner,
>> getting there only 13 years behind the original schedule.
>> (A previous posting of this story was challenged, and Swedish readers
>> produced the following references to support it: "Tiderakning och
>> historia" by Natanael Beckman (1924) and "Tid, en bok om tiderakning
>> och kalendervasen" by Lars-Olof Lode'n (no date was given).)

As for leap seconds, that is something different.  Leap days are ment
to correct for the period of rotation of the earth around the sun
being an non-integral number of days.  Leap seconds are to correct for
the period of rotation on its axis not being exactly 86400 SI seconds.


[*] I'm sure that one of the time nuts actually has, and maybe even
has a web site with the data on it :-)

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