[R] question about seq.dates from chron vs. as.POSIXct

[Gabor Grothendieck]

Yes, its assuming GMT.

I find it best to use an intermediate conversion
to character to avoid these sorts of problems.
If, as in your example, you just have dates and
no times then the following would do it (and has the
advantage that you don't have to specify your
time zone explicitly so it will still work if
someone in another timezone tries your code):

   as.POSIXct(format(as.Date(xt)))

The reason to convert it to Date first, before 
formatting is so that the format will use Date's
default format (which is accepted by as.POSIXct)
rather than chron's default format.

---

Date:   	Wed, 07 Jul 2004 15:30:47 -0500
From:   	Laura Holt <lauraholt_983 at hotmail.com>
To:   	<r-help at stat.math.ethz.ch>
Subject:   	[R] question about seq.dates from chron vs. as.POSIXct

Dear R People:

Here is an interesting question:

>library(chron)
>xt <- seq.dates(from="01/01/2004",by="days",length=5)
>xt
[1] 01/01/04 01/02/04 01/03/04 01/04/04 01/05/04
>
#Fine so far
>as.POSIXct(xt)
[1] "2003-12-31 18:00:00 Central Standard Time"
[2] "2004-01-01 18:00:00 Central Standard Time"
[3] "2004-01-02 18:00:00 Central Standard Time"
[4] "2004-01-03 18:00:00 Central Standard Time"
[5] "2004-01-04 18:00:00 Central Standard Time"
>
Why do the dates change, please? Presumably the as.POSIXct is taking the xt
as midnight GMT and converting to Central Standard Time.

Is the best solution to:
>as.POSIXlt(xt, "CST")
[1] "2004-01-01 CST" "2004-01-02 CST" "2004-01-03 CST" "2004-01-04 CST"
[5] "2004-01-05 CST"
>

Thanks in advance!

Sincerely,
Laura Holt
who is corrupted by dates and times
mailto: lauraholt_983 at hotmail.com