[R] problem setting default timezone

John McKown john.archie.mckown at gmail.com
Thu Apr 23 15:21:29 CEST 2015

On Thu, Apr 23, 2015 at 8:01 AM, Bos, Roger <roger.bos at rothschild.com>

> Dear All,
> I would like to learn the proper way to set the default time zone so I get
> the correct date for my files.  The code below is non-reproducible (sorry)
> because it is based on a file on my system, but I hope someone will be able
> to help me anyway.
> I have a file that was last modified on 4/21/2015:
> > file.info("E:/snap/q/snap_q_q1_" %+% endPeriod %+% ".txt")$mtime
> [1] "2015-04-21 20:26:33 EDT"
> When I convert that to a date, I gives me 2015-04-22.  I read about
> timezones and saw that there are two possible places to set the default
> values: One as a system variable and one as an option.  To be safe I set
> both:
> >     Sys.setenv(TZ='America/New_York')
> >     Sys.getenv("TZ")
> [1] "America/New_York"
> >     options(tz='America/New_York')
> >     getOption("tz")
> [1] "America/New_York"
> >     as.Date(file.info("E:/snap/q/snap_q_q1_" %+% endPeriod %+%
> ".txt")$mtime)
> [1] "2015-04-22"
> But as you can see R still gives me the wrong date.  I can get the correct
> date as follows:
> >     as.Date(file.info("E:/snap/q/snap_q_q1_" %+% endPeriod %+%
> ".txt")$mtime, tz='America/New_York')
> [1] "2015-04-21"
> But my question is why is the as.Date function not using the timezone I
> have set?
> Thank you in advance,
> Roger
​Doing a ?file.info told me that the mtime variable is a POSIXct value.
Doing a ?as.Date told me that when a POSIXct value is given to it, the time
zone defaults to GMT, _not_ to the local time. That is my interpretation of
the documentation.​

The ‘as.Date’ methods accept character strings, factors, logical
     ‘NA’ and objects of classes ‘"POSIXlt"’ and ‘"POSIXct"’.  (The
     last is converted to days by ignoring the time after midnight in
     the representation of the time in specified time zone, default
     UTC.)  Also objects of class ‘"date"’ (from package ‘date’) and
     ‘"dates"’ (from package ‘chron’).  Character strings are processed
     as far as necessary for the format specified: any trailing
     characters are ignored.

​What I would do is:


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