[R] RODBC & POSIX & Daylight Saving blues

Bernie McConnell bm8 at st-andrews.ac.uk
Fri Nov 12 17:27:41 CET 2004

Many thanks for the responses about how to read datetimes into POSIXct 
which refer to straight GMT times without regard to Daylight Saving.  To 
reiterate, I''m using R 2.0.0 on win2000.

I have tried Gabor's suggestion,see below, which did not work for me.  The 
fundamental setting (for me) appears to be in the registry:


The values here are most readily altered using the 'Date/ TIme properties' 
window (just double click the wee digital clock on the right of the window 
task bar).  When I disable Daylight Saving a registry variable 
called  DisableAutoDaylightTimeSet is created and is set to 1.  This can be 
observed with regedit.  Now all works perfectly with my date handling.  So 
it seems that setting the timezone to GMT is necessary but not sufficient 
for my needs

I confess that I do not understand the relationship between setting 
registry variables and environment (eg TZ="GMT").

In practice I would like to keep my win200 machine with Daylight Saving 
enabled - for the sake of other applications.  One strategy is to make the 
required registry alterations immediately before and after any R code which 
handles dates.  But then I'm rather worried whether continuously enabling / 
disabling Daylight Saving would have dire consequences for other 
applications and the well-being of my already shaky OS.  Is this the right 
route - and if so could someone please guide me how to achieve this in R 
for win2000?

Perhaps a better strategy would be to introduce some set-able option within 
R that forced date handling to ignore Daylight Saving.  But here I have 
insufficient expertise and can only appeal to the continuing generosity of 
the developer community.

I am sure that there are many R users in the same position as me - but 
perhaps they don't all realize it!

Many thanks

Bernie McConnell
bm8 at st-andrews.ac.uk
Sea Mammal Reserach Unit
University of St Andrews


Gabor Grothendieck <ggrothendieck <at> myway.com> writes:

: Prof Brian Ripley <ripley <at> stats.ox.ac.uk> writes:

: : If you set it to GMT for the duration of the sqlFetch call, it should do
: : as you intended (but had not told R, which is not clairvoyant).
: :
: On Windows you have to set the whole computer to GMT which has

Paul Roebuck pointed out to me offlist that this can be done
per process in Windows too so I was wrong on this point.

    ... start up Windows console ...
    cd \Program Files\rw2001beta
    set TZ=GMT

In R,

    Sys.time() # displays date and time in GMT time zone

I also tried doing this from within R but was unsuccessful:

    R> Sys.time()
    [1] "2004-11-11 11:37:53 Eastern Standard Time"
    R> Sys.putenv(TZ = "GMT")
    R> Sys.time()  # wanted GMT result but did not get it
    [1] "2004-11-11 11:38:08 Eastern Standard Time"
    R> R.version.string  # Windows XP
    [1] "R version 2.0.1, 2004-11-04"

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