On Thu, 1 Nov 2007, Peter Dalgaard wrote:
[EMAIL PROTECTED] wrote:
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.
Not Our Problem. (This sort of thing never is. We are wholly dependent
on the OS for this information). Check out
http://support.microsoft.com/kb/933360
I think this may well be a long-standing Windows bug that US users have
not had to suffer before. From ?as.POSIXlt
Where OSes describe their valid timezones can be obscure. The
help for 'tzset' (or '_tzset' on Windows) can be helpful, but it
can also be inaccurate. There is a cumbersome POSIX specification
(listed under environment variable 'TZ' at URL:...
which is often at least partially supported, but there may be
other more user-friendly ways to specify timezones. Windows
documents a specification of the form 'GST-1GDT', but seems always
to apply the US rules for changing to/from DST with such a
specification.
Recent versions of Microsoft documentation acknowledge this, e.g.
http://msdn2.microsoft.com/en-us/library/90s5c885(vs.80).aspx
says
dzn
Three-letter daylight-saving-time zone such as PDT. If daylight saving
time is never in effect in the locality, set TZ without a value for
dzn. The C run-time library assumes the United States' rules for
implementing the calculation of daylight saving time (DST).
Now, AFAIK the 'C run-time library' has not been updated to follow US
politics.
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
