[datatable-help] Automagic timezone convertion on data.table rbind

[Short, Tom]

Nicolas, 
 
That's not really a data.table issue. It's generally a problem with
POSIXct. See here:
 
> x = seq(as.POSIXct("2010-01-01", "GMT"), as.POSIXct("2010-01-10",
"GMT"), length.out=2)
> 
> c(x,x)
[1] "2009-12-31 19:00:00 EST" "2010-01-09 19:00:00 EST"
[3] "2009-12-31 19:00:00 EST" "2010-01-09 19:00:00 EST"

To get around this, I generally fix the timezone for the system as
follows:
 
> Sys.setenv(TZ="GMT") # time zones drive me crazy!
> 
> c(x,x)
[1] "2010-01-01 GMT" "2010-01-10 GMT" "2010-01-01 GMT" "2010-01-10 GMT"

Also, if you just have dates, it's better to stick with the Date class
(or chron). In the development version of data.table, we also have
integer date and time classes that work well with data.table (faster
sorting and grouping).
 
- Tom
 



________________________________

	From: datatable-help-bounces at lists.r-forge.r-project.org
[mailto:datatable-help-bounces at lists.r-forge.r-project.org] On Behalf Of
Nicolas Chapados
	Sent: Tuesday, August 31, 2010 15:40
	To: datatable-help at lists.r-forge.r-project.org
	Subject: [datatable-help] Automagic timezone convertion on
data.table rbind
	
	
	Dear list, 
	
	
	I'm using a data.table that contains date columns (in POSIXct
format).  There appears to be an issue with automatic timezone
conversion when rbinding from NULL (or an empty data.table), as would
occur when one is progressively building up the table in a loop.
	
	
	For example:
	
	
	> require(data.table)
	Loading required package: data.table
	
	
	## Currently located in the North-America Eastern Standard Time
zone.
	> Sys.timezone()
	[1] "America/Montreal"
	> a <- NULL
	
	
	## Create a table with a single date column (GMT timezone)
	> b <- data.table(x = seq(as.POSIXct("2010-01-01", "GMT"),
as.POSIXct("2010-01-10", "GMT"), length.out=10))
	> b
	               x
	 [1,] 2010-01-01
	 [2,] 2010-01-02
	 [3,] 2010-01-03
	 [4,] 2010-01-04
	 [5,] 2010-01-05
	 [6,] 2010-01-06
	 [7,] 2010-01-07
	 [8,] 2010-01-08
	 [9,] 2010-01-09
	[10,] 2010-01-10
	
	
	## Bind it from NULL: Oops!  The timezone changes to EST!
	> rbind(a,b)
	                        x
	 [1,] 2009-12-31 19:00:00
	 [2,] 2010-01-01 19:00:00
	 [3,] 2010-01-02 19:00:00
	 [4,] 2010-01-03 19:00:00
	 [5,] 2010-01-04 19:00:00
	 [6,] 2010-01-05 19:00:00
	 [7,] 2010-01-06 19:00:00
	 [8,] 2010-01-07 19:00:00
	 [9,] 2010-01-08 19:00:00
	[10,] 2010-01-09 19:00:00
	
	
	## Same behavior from an empty data table.
	> rbind(data.table(), b)
	                        x
	 [1,] 2009-12-31 19:00:00
	 [2,] 2010-01-01 19:00:00
	 [3,] 2010-01-02 19:00:00
	 [4,] 2010-01-03 19:00:00
	 [5,] 2010-01-04 19:00:00
	 [6,] 2010-01-05 19:00:00
	 [7,] 2010-01-06 19:00:00
	 [8,] 2010-01-07 19:00:00
	 [9,] 2010-01-08 19:00:00
	[10,] 2010-01-09 19:00:00
	
	
	This behavior is somewhat puzzling.  Any pointers as to how to
preserve timezone information would be greatly appreciated!  For the
record, I'm with R version 2.9.2 and time.date version 1.4.1 (the latest
on CRAN).
	
	
	Best regards,
	+ Nicolas Chapados


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.r-forge.r-project.org/pipermail/datatable-help/attachments/20100831/58e127e9/attachment.htm>