[R] Chron, as.POSIXct problem

[Brian Beckage]

Thanks to all who responded to my posting.

At 11:39 AM -0500 11/12/03, Gabor Grothendieck wrote:
>You are being hit by a timezone problem.  Its not really shifting
>the days by one.  Its working in the GMT timezone, not yours.
>
>If you can accept a date format that chron supports then this is the
>easiest solution since chron does not support timezones and so can't
>give you such problems in the first place.  For example,
>the following stays in chron the whole time:
>
>    format(datesTest, format="m/day/year")
>    [1] "Oct/01/1952" "Oct/02/1952" "Oct/03/1952"
>
>If you must convert to POSIXt to take advantage of a format
>only supported by POSIXt then use POSIXlt and specify the timezone explictly:
>
>    format(as.POSIXlt(datesTest,tz="GMT"), "%m/%d/%Y")
>    [1] "10/01/1952" "10/02/1952" "10/03/1952"

This solved the problem using as.POSIXlt().  I guess the tz argument 
doesn't solve the problem using as.POSIXct().  In any case, I'm able 
to use as.POSIXlt() in my current application.

>
>Its because of subtle problems like this that I think that some
>sort of naive (i.e. non-timezone) time such as chron or
>an alternative, should be in the base to encourage wider use.

I agree. Thanks again for your help.

Brian

>
>---
>Date: Wed, 12 Nov 2003 10:05:39 -0500
>From: Brian Beckage <bbeckage at uvm.edu>
>To: <r-help at stat.math.ethz.ch>
>Subject: [R] Chron, as.POSIXct problem
>
>
>
>Dear R list,
>
>I noticed the following 'problem' when changing the format of dates
>created with seq.dates() (from the Chron library) using as.POSIXct()
>(R 1.8.0 on OSX 10.2.8):
>
>>  datesTest<-seq.dates(from="10/01/1952", length=3, by="days");
>>  datesTest
>[1] 10/01/52 10/02/52 10/03/52
>
># Now changing the format to show year as 1952.
>
>>  datesTest<-format(as.POSIXct(datesTest), "%m/%d/%Y")
>>  datesTest
>[1] "09/30/1952" "10/01/1952" "10/02/1952"
>>
>
>The dates were shifted by one day. The work around is simple enough, e.g.,
>
>>  datesTest<-format(as.POSIXct(datesTest+1), "%m/%d/%Y")
>[1] "10/01/1952" "10/02/1952" "10/03/1952"
>
>but I wonder if this is the intended behavior?
>
>Brian
>
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