[R] date calculation

[Ben Bolker]

On 10-10-30 02:02 AM, Shi, Tao wrote:
> Hi Ben,
> 
> That must be the case!  In fact if I do:
> 
>> difftime(strptime("24NOV2004", format="%d%b%Y"), strptime("13MAY2004",format="%d%b%Y"), units="days", tz="GMT")
> Time difference of 195 days
> 
> 
> which supports your claim.
> 
> Can someone from the R development team confirm this?
> 
> Thanks!
> 
> ...Tao
> 
> 

   It sounds like you think this is a bug.  It's not (although it's
arguably not what you want).  The general advice when using dates and
time in R is to use the *least* specific date format that will do what
you want, i.e. don't use a format that incorporates time zone
information (daylight savings time) information if you don't want to
deal with these complexities.

 I would suggest the chron package:

library(chron)
diff(chron(dates.=c("24/11/2004","13/05/2004"),format="d/m/y"))

> 
> 
> 
> ----- Original Message -----
>> From:Ben Bolker <bbolker at gmail.com>
>> To:r-help at stat.math.ethz.ch
>> Cc:
>> Sent:Friday, October 29, 2010 7:54:53 PM
>> Subject:Re: [R] date calculation
>>
>>
>> Shi, Tao <shidaxia <at> 
>> href="http://yahoo.com">yahoo.com> writes:
> 
>> Could someone 
>> explain to me why the following result is not a integer?
>>
>>>
>> difftime(strptime("24NOV2004", format="%d%b%Y"), strptime("13MAY2004", 
>>
>>> format="%d%b%Y"), units="days")
>> Time difference of 195.0417 
>> days
> 
>   Presumably because this goes across a daylight-savings 
>> time
> adjustment?  0.0417=1/24 days is 1 hour ...
> 
>   Ben 
>> Bolker
> 
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