Thank you very much Duncan.
I'll follow your suggestion.
Why do I want to do what the designer did not think anyone would want
to do? I have data acquisition equipment taking measurements every 15
min or so for days at a time, and I need to compile all such
experiments in a master data set. The data acquisition equipment
automatically switches to DST in spring and back to ST in autumn,
which I did not disable because it is easier to work with while we are
running the experiments.
I could use chron to ignore time zones and daylight savings time, but
this would not be of much help as whether or not I use as.POSIXct or
chron, there is one day of the year that has 25 h and I need to deal
with that 25th hour or I'll lose one hour of data!
Denis
Le 09-07-19 à 11:45, Duncan Murdoch a écrit :
On 19/07/2009 11:23 AM, Denis Chabot wrote:
[was " [R] end of daylight saving time"]
Hi,
I got no reply with the previous subject line, probably a bad
choice of subject on my part, so here it is again.
I read from the help on DateTimeClasses and various posts on this
list that, quite logically, one needs to specify if DST is active
or not when time is between 1 and 2 AM on the first Sunday in
November (for North America in recent years).
This I can do for on date at a time:
a <- as.POSIXct("2008-11-02 01:30:00", tz="EST5EDT") # to get
automatic use of DST
b <- as.POSIXct("2008-11-02 01:30:00", tz="EST") # to tell T this
is the second occurrence of 1:30 that day, in ST
difftime(b,a)
Time difference of 1 hours
But why can't I do the following, which appears to be a typical R
way of doing things, to handle several date-times at once?
c <- rep("2008-11-02 01:30:00", 2)
tzone = c("EST5EDT", "EST")
as.POSIXct(c, tz=tzone)
Erreur dans strptime(xx, f <- "%Y-%m-%d %H:%M:%OS", tz = tz) :
valeur 'tz' incorrecte
???
Objects of the POSIXlt and POSIXct classes don't support multiple
time zones, so if you specified several time zones on input, how
would the conversion functions decide which one to use for output?
You'll need to write your own wrapper function to make this
decision, and do the conversions separately for each input timezone.
Why don't those classes support a separate time zone for each entry?
Presumably because their designer never thought anyone would want to
do that.
Duncan Murdoch
Thanks,
Denis Chabot
sessionInfo()
R version 2.9.1 Patched (2009-07-09 r48929)
x86_64-apple-darwin9.7.0
locale:
fr_CA.UTF-8/fr_CA.UTF-8/C/C/fr_CA.UTF-8/fr_CA.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] tools_2.9.1
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