[R] puzzled by time zone quirk

Prof Brian Ripley ripley at stats.ox.ac.uk
Sun Sep 21 16:00:59 CEST 2014

On 21/09/2014 14:11, Denis Chabot wrote:
> Hi,
> I have to deal with time-stamped data coming from outside my own time zone, so the problem is likely poor knowledge of European time zones on my part. But I am puzzled just the same.
> I thought that setting a time zone of "Europe/Copenhagen" would be the same as "CET" in winter and "CEST" in summer.

You thought wrong: CEST is not a valid timezone on most (maybe all) R 

You failed to tell us the 'at a minimum' information required by the 
posting guide.  ?Sys.timezone says OlsonNames() tells you the timezone 
names supported on your unstated platform, and  ?as.POSIXct says

       tz: A time zone specification to be used for the conversion, _if
           one is required_.  System-specific (see time zones), but ‘""’
           is the current time zone, and ‘"GMT"’ is UTC (Universal Time,
           Coordinated). Invalid values are most commonly treated as
           UTC, on some platforms with a warning.

As the posting guide asks, please do your own homework.

> This test in winter works as expected:
>> a = as.POSIXct("2013-02-25 01:00:00", tz="Europe/Copenhagen"); a
> [1] "2013-02-25 01:00:00 CET"
>> b = as.POSIXct("2013-02-25 01:00:00", tz="CET"); b
> [1] "2013-02-25 01:00:00 CET"
>> a-b
> Time difference of 0 secs
> But this one is summer does not work as I expected:
>> c = as.POSIXct("2013-07-25 01:00:00", tz="Europe/Copenhagen"); c
> [1] "2013-07-25 01:00:00 CEST"
>> d = as.POSIXct("2013-07-25 01:00:00", tz="CEST"); d
> [1] "2013-07-25 01:00:00 UTC"
>> e = as.POSIXct("2013-07-25 01:00:00", tz="CET"); e
> [1] "2013-07-25 01:00:00 CEST"
>> c-d
> Time difference of -2 hours
>> c-e
> Time difference of 0 secs
> Setting tz to "Europe/Copenhagen" in summer in c first appears to be the same as setting it to "CEST" because the output is showing "CEST".
> But d should then be the same as c, and it is not.
> What is happening?
> Thanks in advance,
> Denis Chabot
> ______________________________________________
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Emeritus Professor of Applied Statistics, University of Oxford
1 South Parks Road, Oxford OX1 3TG, UK

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