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Re: st: RE: dates
Thank you very much.
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.
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> * http://www.ats.ucla.edu/stat/stata/
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