Re: [boost] Re: reminder about Date/Time formal review

From: William E. Kempf (williamkempf_at_[hidden])
Date: 2002-04-24 09:37:54

----- Original Message -----
From: "Richard Damon" <rdamon_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Tuesday, April 23, 2002 10:15 PM
Subject: RE: [boost] Re: reminder about Date/Time formal review

>
>
> > -----Original Message-----
> > From: boost-admin_at_[hidden] [mailto:boost-admin_at_[hidden]]On
> > Behalf Of Jeff Garland
> > Sent: Tuesday, April 23, 2002 4:27 PM
> > To: boost_at_[hidden]
> > Subject: RE: [boost] Re: reminder about Date/Time formal review
>
> ...
> > > 3.  The Gregorian calendar system needs more documentation.
> > The Gregorian
> > > calendar isn't a simple system, and there are a lot of things
> > not documented
> > > about GDTL's implementation of this system.  For example,
> > historical dates
> > > in a Gregorian system are dependent not only on the date, but
> > also on the
> > > location.  To clarify, different geographical locations adopted the
> > > Gregorian system at different times, and this effects
> > historical dates.  The
> > > two most common "change over dates" used in gregorian date
> > algorithms are
> > > Oct. 4, 1582 and Sept. 14, 1752.  I think it's important to
> > document which
> > > "change over date" is used, since date projections before this
> > date are not
> > > accurate.
> >
> > Agreed.
> >
> > > (Trivia related to the above:  Did you know that Sept. 2, 1752
> > was followed
> > > by Sept. 14, 1752?  Or that George Washington's birthday, which we
give
> > > today as Feb. 22, 1732 was recorded on that day as Feb. 11, 1731?
Gotta
> > > love this date stuff, no?)
> >
>
> Wrong. Gregorian dates are not dependant on where your are in this way.
What
> is dependant is which colander was in use.

July 4, 1751 in the US was a very different day then July 4, 1751 in
England.  The reason, as you point out (and I thought was obvious from what
I posted) is because the US didn't adopt the Gregorian system until Sept.
14, 1752... or, more precisely, they adopted it on Sept. 2, 1752, which
immediately became Sept. 14, 1752.  The point is, you can't project dates
backwards with out knowing the location as well as the date.  Further, the
"change over" date also effects this.  If the change over date used by the
system were the American Sept. 14, 1752 this would mean the algorithm
doesn't take into account things such as the missing day added at this time
when dates are projected further back.

> Gregorian (Sept 14, 1752)-1 = Gregorian(Sept 13, 1752) = Julian(Sept 2,
> 1752)

The above isn't accurate.  Gregorian (Sept. 14, 1752) - 1 = Gregorian (Sept.
12, 1752) if you were in one of the regions that adopted the Gregorian
calendar in 1582, or Julian (Sept. 2, 1742) if you were in America or one of
the other locations that was still using the Julian calendar.  The problem
is that the Gregorian calendar has not been a stable system, with 10 days
being added to the Julian system in 1582 and 11 days being added in 1752
when America switched over.

Bill Kempf