On Mon, 22 Dec 2008, Peter Dalgaard wrote:
Ivan Alves wrote:
Dear all,
Retrieving an Oracle "Date" data type by means of RODBC (version 1.2-4) I
get different classes in R depending on which operating system I am in:
On MacOSX I get "Date" class
On Windows I get " "POSIXt" "POSIXct" class
The problem is material, as converting the "POSIXt" "POSIXct" object with
as.Date() returns one day less ("2008-12-17 00:00:00 CET" is returned as
"2008-12-16").
This is in a sense correct since CET is one hour ahead of GMT (two hours in
Summer). What is a bit puzzling is that
ISOdate(2008,12,24)
[1] "2008-12-24 12:00:00 GMT"
class(ISOdate(2008,12,24))
[1] "POSIXt" "POSIXct"
as.POSIXct("2008-12-24")
[1] "2008-12-24 CET"
as.POSIXct("2008-12-24")+1
[1] "2008-12-24 00:00:01 CET"
I.e. we have two ways of converting a timeless date to POSIXct, and they
differ in noon/midnight, and in whether local timezone matters or not.
I believe Brian did this, and he usually does things for a reason....
Well, one is explicitly a way to set a date, and midday seems a good
choice as most timezones are within +/- 12 hours. OTOH as.POSIXct is
using a format with a missing time, and the POSIX convention is that that
missing times are zero.
The difference is between no time and missing time.
I have 2 related questions:
1. Is there a way to control the conversion used by RODBC for types "Date"?
or is this controlled by the ODBC Driver (in my case the Oracle driver in
Windows and Actual on Mac OS X)?
2. What is the trick to get as.Date() to return the _intended_ date (the
date that the OS X environment "correctly" reads)?
Add 12 hours, maybe? (43200 seconds)
Or play around with the timezone, but that seems painful.
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
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