R help - Apr 2007 - Extent of time zone vulerability for POSIX date and time classes

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).
More importantly, I cannot make chron print the format 12/30/2006 (which 
my output data requires).

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.

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"

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 at 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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