[R] date arithmetic discrepancy

[peter dalgaard]

On Jan 28, 2012, at 07:25 , Berend Hasselman wrote:

> 
> On 27-01-2012, at 21:45, Ty Canuck wrote:
> 
>> What accounts for the different result?
>> 
>>> as.numeric(as.POSIXct("2012-01-13 08:10:00") - as.POSIXct("2012-01-13
>> 00:00:00") , units='secs')
>> [1] 29400
>> 
>>> trunc(as.numeric(as.POSIXct("2012-01-13 08:10:00") -
>> as.POSIXct("2012-01-13 00:00:00") , units='secs') )
>> [1] 29399
> 
> See R FAQ 7.31


More specifically, the "auto" units of difftime() (implied by the subtraction) converts to hours and 29400/3600 cannot be represented exactly in binary.

To retain exact calculations in seconds, set the units directly.

> difftime(as.POSIXct("2012-01-13 08:10:00"), as.POSIXct("2012-01-13 00:00:00"),units="secs")
Time difference of 29400 secs



> 
> And try this
> 
> d1 <- as.numeric(as.POSIXct("2012-01-13 08:10:00") - as.POSIXct("2012-01-13 00:00:00") , units='secs')
> d2 <- trunc(as.numeric(as.POSIXct("2012-01-13 08:10:00") - as.POSIXct("2012-01-13 00:00:00") , units='secs') )
> 
> sprintf("%.12f",d1)
> 
> Berend
> 
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-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com