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/
University of Oxford, Tel: +44 1865 272861 (self)
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and provide commented, minimal, self-contained, reproducible code.
