R devel - Sep 2006 - Date conversion with as.POSIXct and as.POSIXlt (PR#9196)

Full_Name: Erich Neuwirth
Version: 2.3.1
OS: Windows XP, Linux
Submission from: (NULL) (131.130.135.167)


Converting Sys.Date() to a POSIX compliant time type in different ways 
produces inconsistent results:
> Sys.date()
[1] "2006-09-01"
> as.POSIXct(Sys.Date())
[1] "2006-09-01 02:00:00 CEST"
> as.POSIXlt(Sys.Date())
[1] "2006-09-01"
> as.POSIXct(as.POSIXlt(Sys.Date()))
[1] "2006-09-01 01:00:00 CEST"

Applying as.POSIXct directly or first applying as.POSIXlt and then
applying as.POSIXct produces different results.

This happens on Linux with timezone "CEST"
> Sys.time()
[1] "2006-09-01 11:03:36 CEST"
and on Windows with timezone "W. Europe Daylight
Time"
> Sys.time()
[1] "2006-09-01 11:11:15 W. Europe Daylight Time"

erich.neuwirth at univie.ac.at writes:
> Full_Name: Erich Neuwirth
> Version: 2.3.1
> OS: Windows XP, Linux
> Submission from: (NULL) (131.130.135.167)
> 
> 
> Converting Sys.Date() to a POSIX compliant time type in different ways 
> produces inconsistent results:
> > Sys.date()
> [1] "2006-09-01"
> > as.POSIXct(Sys.Date())
> [1] "2006-09-01 02:00:00 CEST"
> > as.POSIXlt(Sys.Date())
> [1] "2006-09-01"
> > as.POSIXct(as.POSIXlt(Sys.Date()))
> [1] "2006-09-01 01:00:00 CEST"
> 
> Applying as.POSIXct directly or first applying as.POSIXlt and then
> applying as.POSIXct produces different results.
> 
> This happens on Linux with timezone "CEST"
> > Sys.time()
> [1] "2006-09-01 11:03:36 CEST"
> and on Windows with timezone "W. Europe Daylight Time"
> > Sys.time()
> [1] "2006-09-01 11:11:15 W. Europe Daylight Time"

Or,
> as.POSIXct(Sys.Date()) - as.POSIXlt(Sys.Date())
Time difference of 1 hours
> as.POSIXct(Sys.time()) - as.POSIXlt(Sys.time())
Time difference of -0.0003409386 secs

Whereas
> Sys.putenv("TZ"="GMT")
> as.POSIXct(Sys.Date()) - as.POSIXlt(Sys.Date())
Time difference of 0 secs

and
> Sys.putenv("TZ"="EST")
> as.POSIXct(Sys.Date()) - as.POSIXlt(Sys.Date())
Time difference of -5 hours

So the difference is that as.POSIXct works in GMT whereas as.POSIXlt
works in the local timezone w/o DST corrections, the point of the
latter likely being that
>
as.POSIXlt(as.Date("2006-7-1"))-as.POSIXlt(as.Date("2006-1-1"))
Time difference of 181 days

comes out as an integer number of days.

BTW, the habit of format.POSIXlt dropping off the time when it is
exactly 01:00 in CEST (i.e. would have been midnight, save for
Daylight Savings Time) is a bit confusing too...  

I'm not sure any of this is actual bugs (Brian will tell us soon
enough), but it does appear at least to be underdocumented.

-- 
   O__  ---- Peter Dalgaard             ?ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

You misunderstand the role of timezones.

as.POSIXct(Sys.Date())

is in UTC, as documented.

as.POSIXlt(Sys.Date())

does not have a timezone (POSIXlt objects do not need one).

as.POSIXct(as.POSIXlt(Sys.Date()))

means

as.POSIXct(as.POSIXlt(Sys.Date()), tz="")

which is asking to interpret this in the local timezone.

I've expanded the documentation, but I see no sign you had studied it so 
that it unlikely to help you.

On Fri, 1 Sep 2006, erich.neuwirth at univie.ac.at wrote:
> Full_Name: Erich Neuwirth
> Version: 2.3.1
> OS: Windows XP, Linux
> Submission from: (NULL) (131.130.135.167)
> 
> 
> Converting Sys.Date() to a POSIX compliant time type in different ways 
> produces inconsistent results:
> > Sys.date()
> [1] "2006-09-01"
> > as.POSIXct(Sys.Date())
> [1] "2006-09-01 02:00:00 CEST"
> > as.POSIXlt(Sys.Date())
> [1] "2006-09-01"
> > as.POSIXct(as.POSIXlt(Sys.Date()))
> [1] "2006-09-01 01:00:00 CEST"
> 
> Applying as.POSIXct directly or first applying as.POSIXlt and then
> applying as.POSIXct produces different results.
> 
> This happens on Linux with timezone "CEST"
> > Sys.time()
> [1] "2006-09-01 11:03:36 CEST"
> and on Windows with timezone "W. Europe Daylight Time"
> > Sys.time()
> [1] "2006-09-01 11:11:15 W. Europe Daylight Time"
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Note that there is some discussion in the help desk article
in R News 4/1 and a table of tested conversions at
the end of that article.    If x is of class "Date" then for
converting from Date to POSIXct the table suggests
as.POSIXct(format(x)) or as.POSIXct(format(x), tz = "GMT")
according to whether you want the current time zone or GMT.

On 9/1/06, erich.neuwirth at univie.ac.at <erich.neuwirth at univie.ac.at>
wrote:
> Full_Name: Erich Neuwirth
> Version: 2.3.1
> OS: Windows XP, Linux
> Submission from: (NULL) (131.130.135.167)
>
>
> Converting Sys.Date() to a POSIX compliant time type in different ways
> produces inconsistent results:
> > Sys.date()
> [1] "2006-09-01"
> > as.POSIXct(Sys.Date())
> [1] "2006-09-01 02:00:00 CEST"
> > as.POSIXlt(Sys.Date())
> [1] "2006-09-01"
> > as.POSIXct(as.POSIXlt(Sys.Date()))
> [1] "2006-09-01 01:00:00 CEST"
>
> Applying as.POSIXct directly or first applying as.POSIXlt and then
> applying as.POSIXct produces different results.
>
> This happens on Linux with timezone "CEST"
> > Sys.time()
> [1] "2006-09-01 11:03:36 CEST"
> and on Windows with timezone "W. Europe Daylight Time"
> > Sys.time()
> [1] "2006-09-01 11:11:15 W. Europe Daylight Time"
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>