On Sat, 24 Sep 2011, Folkes, Michael wrote:

Hello all,
After beating my head against the wall for a few hours, I give up.
I have two problems.
1. If I use seq() to generate a POSIXct series that crosses a daylight savings 
to stardard time change I get two values for the first day of change.  The time 
change is Nov 6,2011.  The value for Nov 7 turns into 23:00 on Nov 6, thus two 
values for Nov 6.  (see below)
So I put a time value (noon) into the sequence, that did give me a correct date 
sequence.

That is as documented. Did you actually read the help page, which says

     The difference between ‘"day"’ and ‘"DSTday"’ is that the former
     ignores changes to/from daylight savings time and the latter takes
     the same clock time each day.

So you asked to increment the clock by 24 hours and not to the same time the next day. I cannot reproduce your results as you have not told us your timezone, but I can guess America/Vancouver

seq(start,end,by='day')
[1] "2011-11-05 00:00:00 PDT" "2011-11-06 00:00:00 PDT"
[3] "2011-11-06 23:00:00 PST"
seq(start,end,by='DSTday')
[1] "2011-11-05 PDT" "2011-11-06 PDT" "2011-11-07 PST"



2. When using that modified date sequence in maptools::sunriset(), on day of 
change to stardard time the sunrise time is shifted by one hour. (i.e. Nov5 
8am, Nov6 6am, Nov7 7am)
I can't fathom why sunriset() is giving me funny results on the day of time 
change.  I wonder if there is something amiss with how my windows XP is 
managing time zones perhaps? I would have expected sunriset() to be using just 
the day value from its dateTime argument, not the time component?
thanks!
Michael

__________________________

start <- as.POSIXct("2011-11-05")
end <- as.POSIXct("2011-11-07")
date.ser<-seq(start,end,by='day')
# time change is Nov 6,2011
date.ser
#####  Two values for Nov 6 as Nov 7th lost an hour.
[1] "2011-11-05 00:00:00 PDT" "2011-11-06 00:00:00 PDT"
[3] "2011-11-06 23:00:00 PST"


### Add noon to correct flexibility for time shift.
start <- as.POSIXct("2011-11-05 12:00:00")
end <- as.POSIXct("2011-11-07 12:00:00")
date.ser<-seq(start,end,by='day')
date.ser
[1] "2011-11-05 12:00:00 PDT" "2011-11-06 11:00:00 PST"
[3] "2011-11-07 11:00:00 PST"

library(maptools)
nanaimo <- matrix(c(-123+(-57/60),49+11/60), nrow=1)
sunriset(nanaimo, date.ser, direction="sunrise", POSIXct.out=T)[,2]
[1] "2011-11-05 08:09:42 PDT" "2011-11-06 06:11:19 PST"
[3] "2011-11-07 07:12:56 PST"

_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  michael.fol...@dfo-mpo.gc.ca


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Brian D. Ripley,                  rip...@stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
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