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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: RE: dates |

Date |
Thu, 21 Aug 2003 13:31:52 +0100 |

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 * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: RE: dates***From:*Ulrich Kohler <kohler@wz-berlin.de>

**References**:**st: dates***From:*Ulrich Kohler <kohler@wz-berlin.de>

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