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

[Peter Dalgaard]

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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>   


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