R devel - Nov 2006 - as.Date: conversion pb from POSIXct (PR#9386)

Full_Name: Xiao Gang FAN
Version: 2.4.0
OS: Windows
Submission from: (NULL) (159.50.101.9)

> library(chron)
> as.Date(as.POSIXct(strptime("1994-01-24","%Y-%m-%d")))
[1] "1994-01-23"

as.Date is converting your object with respect to GMT time zone and in
that time zone your POSIXct date is the day before.  See ?as.Date and also
suggest you read R News 4/1 help desk article.


On 11/27/06, xiao.gang.fan1 at free.fr <xiao.gang.fan1 at free.fr>
wrote:
> Full_Name: Xiao Gang FAN
> Version: 2.4.0
> OS: Windows
> Submission from: (NULL) (159.50.101.9)
>
>
> > library(chron)
> >
as.Date(as.POSIXct(strptime("1994-01-24","%Y-%m-%d")))
> [1] "1994-01-23"
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

You have not told us your timezone, but this seems to be as documented if 
you are in CET.  The help page says

      The 'as.Date' methods accept character strings, factors, logical
      'NA' and objects of classes '"POSIXlt"' and
'"POSIXct"'.  (The
      last are converted to days by ignoring the time after midnight in
      the representation of the time in UTC.)
                                     ^^^^^^

Probably as.POSIXct(strptime("1994-01-24","%Y-%m-%d") is
1994-01-23 23:00
in UTC in your timezone.

On Mon, 27 Nov 2006, xiao.gang.fan1 at free.fr wrote:
> Full_Name: Xiao Gang FAN
> Version: 2.4.0
> OS: Windows
> Submission from: (NULL) (159.50.101.9)
>
>
>> library(chron)
Irrelevant.
>>
as.Date(as.POSIXct(strptime("1994-01-24","%Y-%m-%d")))
> [1] "1994-01-23"
Not in my timezone.

-- 
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