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Re: st: RE: dates


From   Ulrich Kohler <kohler@wz-berlin.de>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: dates
Date   Thu, 21 Aug 2003 15:16:58 +0200

Nick, 

Thank you very much. 
uli

Nick Cox wrote:
> Ulrich Kohler
>
> > I have two  questions on elapsed dates:
> >
> > The mdy()-function returns the elapsed date, counting from
> > January 1 1960.
> >
> > . display mdy(1,1,1960)
> > 0
> >
> > . display mdy(1,2,1960)
> > 1
> >
> > . display mdy(12,24,100)
> > -678993
> >
> > However
> >
> > . display mdy(12,24,99)
> >
> > returns a missing value, as does
> >
> > . display mdy(12,24,00)
> > .
> >
> > What is the reason that mdy() returns missing with 2 digit
> > years ( to prevent
> > the user from using two digit years?)
>
> Dates in Stata start at 1 January 100. That once put paid
> to a project I wanted to do on modelling of
> daily temperature measurements and olive prices under
> Julius Caesar, but Stata Corp were unyielding: they have
> no interest in changing this limit.
>
> As you say, elsewhere you can indicate separately a two-digit
> year and a century, so that "52" and "19" can be coupled
> together. It gets a bit awkward to imagine "52" and "0"
> being coupled together to indicate the year 52.
>
> > The second question is just for curiosity. Is there any
> > reason for using the
> > January 1 1960 as zero? It seems quite common in software
> > programs, but why?
>
> Interesting little historical question. I guess some
> data base or spreadsheet program started this as a
> convention, and lots followed. Perhaps there was a
> presumption that nobody much wanted to deal with
> data before that. (Depending upon implementation,
> perhaps you could have dates before as negative numbers,
> as in Stata.)
>
> Also, suppose you used 1 January 1 as a basis. Then
> most daily dates which would be used would be represented
> by integers ~ 700,000 and that could tie up lots of bytes which,
> a few years ago, was of course a much bigger deal.
>
> As far as Stata is concerned, a more natural base
> would be 21 January 1952 (see [U] p.302), but that
> would be rather too idiosyncratic to fit the bill.
>
> Nick
> n.j.cox@durham.ac.uk
>
>
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-- 
kohler@wz-berlin.de


*
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