[R] Extent of time zone vulerability for POSIX date and time classes

[Tim Bergsma]

Gabor,

thanks for the feedback. I like the idea of coercing the system time 
zone, and sticking with strptime/strftime.  The balance would probably 
tip in favor of chron, however, if I could get the format correct. 
Apparently the default is two-digit year:

library(chron)
chron(1)
[1] 01/02/70

chron(1,format="m/d/y")
[1] 01/02/70

But I need four digit year, so I try:

chron(1,format="m/d/yyyy")
[1] Jan/02/1970

Now, month has switched to character!  I don't know of a format 
convention that gets it back to numeric (although conventions are 
documented for forcing to character).  Comment?

Regards,

Tim.


Gabor Grothendieck wrote:
> If you use
> 
> Sys.putenv(TZ = "GMT")
> 
> at the beginning of your session then local time zone and GMT time
> zone will be the same so you should not have a problem.  This was
> not possible, at least on Windows, at the time the R News article
> was written.
> 
> 
> 
> 
> 
> On 4/5/07, Tim Bergsma <timb at metrumrg.com> wrote:
>> Hi.
>>
>> I frequently convert date and time data to and from character
>> representations.  I'm frustrated with chron, because 'seconds' are
>> required to create a time object (my input data never has seconds).
> 
> You could use paste:
> 
>  times(paste("12:30", 0, sep = ":"))
> 
> and make it into a function if you use it a lot.
> 
>> More importantly, I cannot make chron print the format 12/30/2006 (which
>> my output data requires).
> 
> That is the default output format so you don't have to specify anything. 
> Its
> only with POSIX that its not the default.  For example:
> 
>> library(chron)
>> x <- chron("12/30/2006")
>> x
> [1] 12/30/06
> 
>>
>> I really like the format flexibility of strftime() and strptime(), but
>> of course am paranoid about timezone issues.  After reading the standard
>> reference several times
>> (http://cran.r-project.org/doc/Rnews/Rnews_2004-1.pdf), I am tempted to
>> conclude that if I never specify timezones, and never use Sys.time(),
>> the vulnerabilities do not pertain.
> 
> That won't protect you but this would let you use POSIX safely:
> 
> http://www.mail-archive.com/r-help@stat.math.ethz.ch/msg83508.html
> 
>>
>> To the point: if I'm merely converting to and from character data that
>> does not represent time zones, is there still a time zone vulnerability
>> with strftime() and strptime()?
>>
>> Thanks in advance,
>>
>> Tim Bergsma, PhD
>> Metrum Research Group, LLC
>>
>>
>> #example
>> strftime(
>>   strptime(
>>     "30-Dec-06 23:30",
>>     format="%d-%b-%y %H:%M"
>>   ),
>>   format="%m/%d/%Y %H:%M"
>> )
>> [1] "12/30/2006 23:30"
>>
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> 
>