[R] time serie : question about date time class

[Laurent Rhelp]

jim holtman a écrit :

>Depending on how you are using POSIXct, you accuracy is limited to the
>microsecond level.  It is stored as a floating point number with 54
>bit of accuracy (~16 digits) and currently the number of seconds since
>1/1/1970 is 10 digits, so with microseconds adding 6 more, you are at
>the limit:
>
>  
>
>>x <- Sys.time()
>>x
>>    
>>
>[1] "2010-02-06 09:34:58 EST"
>  
>
>>unclass(x)
>>    
>>
>[1] 1265466899
>  
>
>>x
>>    
>>
>[1] "2010-02-06 09:34:58 EST"
>  
>
>>y <- x+.000001
>>x-y
>>    
>>
>Time difference of -9.536743e-07 secs
>  
>
>>y <- x+.0000001
>>x-y
>>    
>>
>Time difference of 0 secs
>  
>
>>y <- x+.000001  # 1 us
>>identical(x,y)
>>    
>>
>[1] FALSE
>  
>
>>y <- x+.0000001  # 0.1 us
>>identical(x,y)
>>    
>>
>[1] TRUE
>  
>
>
>
>On Sat, Feb 6, 2010 at 9:41 AM, Laurent Rhelp <LaurentRHelp at free.fr> wrote:
>  
>
>>Gabor Grothendieck a écrit :
>>
>>    
>>
>>>zoo is independent of time and date class so it does not restrict your
>>>choice of index class.  POSIXct supports sub-microsecond accuracy.
>>>See ?POSIXct .  Simply using the number of microseconds since the
>>>start of the experiment is another possibility.
>>>
>>>On Sat, Feb 6, 2010 at 8:25 AM, Laurent Rhelp <LaurentRHelp at free.fr>
>>>wrote:
>>>
>>>      
>>>
>>>>Dear R-List,
>>>>
>>>>I have the habit of using R for my data processing and I like to use the
>>>>power of the lattice package. Presently, I have to manage time series.
>>>>So,
>>>>in order to work properly I want to discover the zoo package and the
>>>>related
>>>>methods (since lattice can work with zoo class). But my physical
>>>>experiment
>>>>lasts only one second and my sampling period is equal to 1 microsecond
>>>>(the
>>>>date time value is given by the IRIG Card for my data acquisition card).
>>>>I
>>>>read the R-News june 2004 about the date time Classes in R and there is
>>>>no
>>>>information about allowing for the microseconds. So do you think it is
>>>>really a good idea to try to use the R time series framework (zoo package
>>>>for example) with my data ? Or would there be a tip ?
>>>>
>>>>Thank you very much
>>>>
>>>>Laurent
>>>>
>>>>______________________________________________
>>>>R-help at r-project.org mailing list
>>>>https://stat.ethz.ch/mailman/listinfo/r-help
>>>>PLEASE do read the posting guide
>>>>http://www.R-project.org/posting-guide.html
>>>>and provide commented, minimal, self-contained, reproducible code.
>>>>
>>>>
>>>>        
>>>>
>>>      
>>>
>>Thank you, I will see in details the POSIXct class.
>>
>>______________________________________________
>>R-help at r-project.org mailing list
>>https://stat.ethz.ch/mailman/listinfo/r-help
>>PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>and provide commented, minimal, self-contained, reproducible code.
>>
>>    
>>
>
>
>
>  
>
This is a good piece of advice ! Thank you very much, I will take care 
of it  in my calculations.