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

[tplate at acm.org]

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.

-- Tony Plate