as.POSIXct assigns different timezones

Question

I am importing a simple timestamp file with the structure "%Y-%m-%d %H:%M:%S" , i.e. "2014-10-26 04:15:32". A sample data set:

tz <- c("2014-10-26 00:27:14", "2014-10-26 00:42:14", "2014-10-26 00:57:14", "2014-10-26 01:12:14", "2014-10-26 01:27:14", "2014-10-26 01:42:14", "2014-10-26 01:57:14", "2014-10-26 02:00:32", "2014-10-26 02:12:14", "2014-10-26 02:15:32", "2014-10-26 02:27:14", "2014-10-26 02:30:32", "2014-10-26 02:42:14", "2014-10-26 02:45:32", "2014-10-26 02:57:14", "2014-10-26 03:00:32", "2014-10-26 03:15:32", "2014-10-26 03:30:32", "2014-10-26 03:45:32", "2014-10-26 04:00:32", "2014-10-26 04:15:32" )

Afterwards, I use tz <- as.POSIXct(tz, format="%Y-%m-%d %H:%M:%S") without assigning any timezone. However, when I display tz the output is the following:

[1] "2014-10-26 00:27:14 CEST" "2014-10-26 00:42:14 CEST" "2014-10-26 00:57:14 CEST" "2014-10-26 01:12:14 CEST" [5] "2014-10-26 01:27:14 CEST" "2014-10-26 01:42:14 CEST" "2014-10-26 01:57:14 CEST" "2014-10-26 02:00:32 CEST" [9] "2014-10-26 02:12:14 CEST" "2014-10-26 02:15:32 CEST" "2014-10-26 02:27:14 CEST" "2014-10-26 02:30:32 CET" [13] "2014-10-26 02:42:14 CET" "2014-10-26 02:45:32 CET" "2014-10-26 02:57:14 CET" "2014-10-26 03:00:32 CET" [17] "2014-10-26 03:15:32 CET" "2014-10-26 03:30:32 CET" "2014-10-26 03:45:32 CET" "2014-10-26 04:00:32 CET" [21] "2014-10-26 04:15:32 CET"

Any ideas why R assigns two different time zones - CEST and CET? I changed my local settings to Sys.setlocale("LC_ALL","English") and I also used Sys.setenv(TZ='CET') but without any result. I used different ways of conversion described Change timezone in a POSIXct object and http://blog.revolutionanalytics.com/2009/06/converting-time-zones.html but I am still not able to change the timezone to be only one. Thanks in advance!

Oh snap! The time changed exactly on 26th Oct and I have the same problem on 29th March where the time changed again. Any ideas how to avoid this? — Boris Nikolov, Jul 26 '15 at 16:33
CET or CEST, it doesn't really matter, I just want to keep it consistent through the whole dataset. — Boris Nikolov, Jul 26 '15 at 17:05
CET and CEST *are* the same time zone. What you see is daylight saving time. — Joshua Ulrich, Jul 26 '15 at 17:54
Yes but some countries change to CEST and some don't. How can I handle this? — Boris Nikolov, Jul 26 '15 at 18:07
Then set the timezone to one that doesn't observe daylight saving time. You don't have to use your operating system's timezone. See `?timezone`. — Joshua Ulrich, Jul 26 '15 at 18:25
I tried using Europe/Paris but it's the same. I guess I have to choose a country which does not change the time. Thank you all! — Boris Nikolov, Jul 26 '15 at 18:59
I suppose you could use that strategy. Seems simpler to use the strategy in the answer to the duplicate question, though. — IRTFM, Jul 27 '15 at 01:00

as.POSIXct assigns different timezones

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