[Rd] daylight saving / time zone issues with as.POSIXlt/as.POSIXct (PR#10392)

From: <tplate_at_acm.org>
Date: Thu, 1 Nov 2007 17:00:09 +0100 (CET)


Running under Windows XP 64 bit, as.POSIXlt()/as.POSIXct() seem to think that US time zones (EST5EDT, MST7MDT) switched from daylight savings back to standard time on Oct 28, 2007, whereas the switch is actually on Sun Nov 04, 2007.

Examples:

 > Sys.timezone()
[1] "Mountain Daylight Time"
 > as.POSIXct("2007-10-30 12:38:47")
[1] "2007-10-30 12:38:47 Mountain Daylight Time"
 > # *** Should report 2007-10-30 14:38:47 EDT:  > as.POSIXlt(as.POSIXct("2007-10-30 12:38:47"), "EST5EDT") [1] "2007-10-30 13:38:47 EST"
 > Sys.time()
[1] "2007-11-01 09:22:28 Mountain Daylight Time"

 > # Bad behavior is manifested in different ways with TZ="MST7MDT"
 > Sys.setenv(TZ="MST7MDT")
 > # *** Should report "12:38:47 MDT"
 > as.POSIXct("2007-10-30 12:38:47")

[1] "2007-10-30 12:38:47 MST"
 > as.POSIXlt(as.POSIXct("2007-10-30 12:38:47"), "EST5EDT") [1] "2007-10-30 14:38:47 EST"
 > # *** Should report "2007-11-01 09:23:09 MDT"  > Sys.time()

[1] "2007-11-01 08:23:09 MST"
 >
 > sessionInfo()
R version 2.6.0 Patched (2007-10-11 r43143) i386-pc-mingw32

locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United States.1252;LC_MONETARY=English_United States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252

attached base packages:
[1] stats graphics grDevices utils datasets methods base  >

Furthermore, with the timezone "Mountain Daylight Time" (which is the default I get when I start R), the switch appears to be on Nov 5 in 2006, whereas it actually was on Oct 29 in 2006.

 > # New R session
 > Sys.timezone()
[1] "Mountain Daylight Time"
 > # *** wrong switch in 2006 ***
 > as.POSIXct("2006-10-30 12:38:47")+(-4:7)*(24*3600)
[1] "2006-10-26 12:38:47 Mountain Daylight Time"
[2] "2006-10-27 12:38:47 Mountain Daylight Time"
[3] "2006-10-28 12:38:47 Mountain Daylight Time"
[4] "2006-10-29 12:38:47 Mountain Daylight Time"
[5] "2006-10-30 12:38:47 Mountain Daylight Time"
[6] "2006-10-31 12:38:47 Mountain Daylight Time"
[7] "2006-11-01 12:38:47 Mountain Daylight Time"
[8] "2006-11-02 12:38:47 Mountain Daylight Time"
[9] "2006-11-03 12:38:47 Mountain Daylight Time"

[10] "2006-11-04 12:38:47 Mountain Daylight Time"
[11] "2006-11-05 11:38:47 Mountain Standard Time"
[12] "2006-11-06 11:38:47 Mountain Standard Time"
 > as.POSIXct("2007-10-30 12:38:47")+(-4:7)*(24*3600)
[1] "2007-10-26 12:38:47 Mountain Daylight Time"
[2] "2007-10-27 12:38:47 Mountain Daylight Time"
[3] "2007-10-28 12:38:47 Mountain Daylight Time"
[4] "2007-10-29 12:38:47 Mountain Daylight Time"
[5] "2007-10-30 12:38:47 Mountain Daylight Time"
[6] "2007-10-31 12:38:47 Mountain Daylight Time"
[7] "2007-11-01 12:38:47 Mountain Daylight Time"
[8] "2007-11-02 12:38:47 Mountain Daylight Time"
[9] "2007-11-03 12:38:47 Mountain Daylight Time"
[10] "2007-11-04 11:38:47 Mountain Standard Time"
[11] "2007-11-05 11:38:47 Mountain Standard Time"
[12] "2007-11-06 11:38:47 Mountain Standard Time"
 > Sys.setenv(TZ="MST7MDT")
 > Sys.timezone()
[1] "MST"
 > as.POSIXct("2006-10-30 12:38:47")+(-4:7)*(24*3600)
[1] "2006-10-26 13:38:47 MDT" "2006-10-27 13:38:47 MDT"
[3] "2006-10-28 13:38:47 MDT" "2006-10-29 12:38:47 MST"
[5] "2006-10-30 12:38:47 MST" "2006-10-31 12:38:47 MST"
[7] "2006-11-01 12:38:47 MST" "2006-11-02 12:38:47 MST"
[9] "2006-11-03 12:38:47 MST" "2006-11-04 12:38:47 MST"
[11] "2006-11-05 12:38:47 MST" "2006-11-06 12:38:47 MST"  > # *** wrong switch in 2007 ***
 > as.POSIXct("2007-10-30 12:38:47")+(-4:7)*(24*3600)
[1] "2007-10-26 13:38:47 MDT" "2007-10-27 13:38:47 MDT"
[3] "2007-10-28 12:38:47 MST" "2007-10-29 12:38:47 MST"
[5] "2007-10-30 12:38:47 MST" "2007-10-31 12:38:47 MST"
[7] "2007-11-01 12:38:47 MST" "2007-11-02 12:38:47 MST"
[9] "2007-11-03 12:38:47 MST" "2007-11-04 12:38:47 MST"
[11] "2007-11-05 12:38:47 MST" "2007-11-06 12:38:47 MST"  >

I see this behavior on all the Windows systems I have tried: Windows XP 64 bit, Windows XP 32 bit Pro, Windows XP home, Windows 2000, with a variety of R versions. The systems have all relevant Windows updates applied (unless some were inadvertently missed) and the systems otherwise appear to behave correctly with respect to times and timezones.

I do not see this problem on Ubuntu Linux systems.


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