Re: [R] How to convert Datetime numbers from Excel to POSIXt objects

From: Peter Dalgaard <P.Dalgaard_at_biostat.ku.dk>
Date: Fri, 14 Dec 2007 17:23:55 +0100

Rolf Fankhauser wrote:
> Hi all,
>
> I need to compare time series data files of different time formats. I
> had no problems with text format using strptime.
> But how can I convert datetime numbers from Excel (days since 30.12.1899
> 00:00:00) into POSIXt objects?
> For example 29770.375 should be converted to "03.07.1981 09:00:00"
>
> I tried the following code and encountered strange results:
>
> t1-t0 gives 29770.33 (should be 29770.375 in my opinion)
> t1-t2 and t1-t3 are ok
> t1-t4 gives 183.3333 (should be 183.375)
> Are these rounding errors?
>
> 1/(.375 - .333333)
[1] 23.99981

So your expectation is off by 1/24th of a day.

Can you think of something that might affect time differences by that amount, depending on which times of the year you are comparing?

> The R-code:
>
> t1 <- strptime("3.7.1981 09:00:00","%d.%m.%Y %H:%M:%S")
> t0 <- strptime("30.12.1899 00:00:00","%d.%m.%Y %H:%M:%S")
> t2 <- strptime("3.7.1981 00:00:00","%d.%m.%Y %H:%M:%S")
> t3 <- strptime("1.7.1981 00:00:00","%d.%m.%Y %H:%M:%S")
> t4 <- strptime("1.1.1981 00:00:00","%d.%m.%Y %H:%M:%S")
> t1 - t0
> t1 - t2
> difftime(t1,t2,units="days")
> t1 - t3
> t1 - t4
>
> Thanks for any help or clarifications
>
> Rolf
>
> ______________________________________________
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-- 
   O__  ---- Peter Dalgaard             Ă˜ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard_at_biostat.ku.dk)                  FAX: (+45) 35327907

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Received on Fri 14 Dec 2007 - 16:33:49 GMT

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