From diggsb at ohsu.edu  Thu Feb  4 22:39:43 2010
From: diggsb at ohsu.edu (Brian Diggs)
Date: Thu, 4 Feb 2010 13:39:43 -0800
Subject: [Rd] Error with cut.POSIXt and daylight savings time switchover
 dates (patch included)
In-Reply-To: <E5BA65CAFB491A4DBB1370F6C972165502AEAD7498@EX-MB05.ohsu.edu>
References: <E5BA65CAFB491A4DBB1370F6C972165502AE890131@EX-MB05.ohsu.edu>
	<E5BA65CAFB491A4DBB1370F6C972165502AEAD7498@EX-MB05.ohsu.edu>
Message-ID: <E5BA65CAFB491A4DBB1370F6C972165502AEAD76F3@EX-MB05.ohsu.edu>

On 2/3/2010 7:48 AM, Brian Diggs wrote:
> On 2/1/2010 3:57 PM, Brian Diggs wrote:
>> The following code:
>>
>> cut(as.POSIXct("2009-11-01 04:00:00", tz="America/Los_Angeles"), "1 day")
>>
>> gives the error:
>>
>> Error in seq.int(0, to - from, by) : 'to' must be finite

[details deleted...]

>> Is there a timezone/daylight savings time safe way to get to the
>> beginning of the day in cut.POSIXt so that invalid dates are not sent
>> to the other functions?  Alternatively, can as.POSIXct.POSIXlt be
>> made to handle these manually manipulated dates correctly?
> 
> I realized I forgot my session info:
> 
>> sessionInfo()
> R version 2.10.1 (2009-12-14) 
> i386-pc-mingw32 
> 
> locale:
> [1] LC_COLLATE=English_United States.1252 
> [2] LC_CTYPE=English_United States.1252   
> [3] LC_MONETARY=English_United States.1252
> [4] LC_NUMERIC=C                          
> [5] LC_TIME=English_United States.1252    
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base     
> 
> 
>> -- Brian Diggs, Ph.D. Senior Research Associate, Department of
>> Surgery, Oregon Health & Science University
> 

A post by Felix Andrews in a different thread pointed out the trunc.POSIXt function.  I checked, and it properly handles the daylight savings time issue:

tm <- as.POSIXlt("2009-11-01 04:00:00", tz="America/Los_Angeles")
tm <- trunc(tm, "days")
as.POSIXct(tm)
# [1] "2009-11-01 PDT"

Looking at how trunc.POSIXt does this, I created a patch (included below) for cut.POSIXt  so that it won't break either.  I've not extensively tested this patch (I don't have the R build environment for Windows installed on my machine), but being analogous to existing base code, it should work generally.


Index: datetime.R
===================================================================
--- datetime.R	(revision 51101)
+++ datetime.R	(working copy)
@@ -729,7 +729,7 @@
 	incr <- 1
 	if(valid > 1L) { start$sec <- 0L; incr <- 59.99 }
 	if(valid > 2L) { start$min <- 0L; incr <- 3600 - 1 }
-	if(valid > 3L) { start$hour <- 0L; incr <- 86400 - 1 }
+	if(valid > 3L) { start$hour <- 0L; start$isdst <- -1L; incr <- 86400 - 1 }
 	if(valid == 5L) { # weeks
 	    start$mday <- start$mday - start$wday
 	    if(start.on.monday)



[Rd] Error with cut.POSIXt and daylight savings time switchover dates (patch included)
Brian Diggs