[Rd] Date conversion with as.POSIXct and as.POSIXlt (PR#9196)

[Gabor Grothendieck]

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"
>
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