time zones, daylight saving etc.

Hi,

seq.dates() in the chron package does not allow creating sequences by
minutes, so you'd have to roll your own sequence generator.

Looks like the tzone attribute of the times is lost when using min(),
max() and seq().  You can apply it back manually, but it does not
affect the calculation, since POSIXct times are stored as seconds
since 1/1/1970 (?DateTimeClasses).

## These dates/times just span the move from NZDT to NZST:
dt.dates <- paste(rep(15:16, c(5,7)), "03", "2003", sep="/")
dt.times <- paste(c(19:23, 0:6), "05", sep=":")
dt <- paste(dt.dates, dt.times)

## No shift in times, or worrying about daylight savings; appropriate
## iff the device doing the recording was not itself adjusting for
## daylight savings, presumably.
datetime <- as.POSIXct(strptime(dt, "%d/%m/%Y %H:%M"), "GMT")

## Create two objects with all the times in your range, one with the
## tzone attribute set back to GMT (to match datetimes), and one
## without this.
mindata1 <- mindata2 <- seq(from=min(datetime), to=max(datetime),
                           by="mins")
attr(mindata2, "tzone") <- "GMT"

fmt <- "%Y %m %d %H %M"
## These both do the matching correctly:
match(format(datetime, fmt), format(mindata1, fmt, tz="GMT"))
match(format(datetime, fmt), format(mindata2, fmt, tz="GMT"))

## However, the first of these will not, as it gets the timezone all
## wrong, since it's neither specified in the call to format(), or as
## an attribute of the POSIXct object.
match(format(datetime, fmt), format(mindata1, fmt))
match(format(datetime, fmt), format(mindata2, fmt))

## It is also possible to run match() directly off the POSIXct object,
## but I'm not sure how this will interact with things like leap
## seconds:
match(datetime, mindata1)

Time zones do my head in, so you probably want to check this all
pretty carefully.  Looks like there's lots of gotchas (e.g. subsetting
a POSIXct object strips the tzone attribute).

Cheers,
Rich

On 5/12/05, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:

You could use the chron package.  It represents date times without
using time zones so you can't have this sort of problem.

On 5/10/05, Carla Meurk <ksm32 at student.canterbury.ac.nz> wrote:

Hi,  I have a whole bunch of data, which looks like:

15/03/2003       10:20  1
15/03/2003       10:21  0
15/03/2003       12:02  0
16/03/2003       06:10  0
16/03/2003       06:20  0.5
16/03/2003       06:30  0
16/03/2003       06:40  0
16/03/2003       06:50  0

18/03/2003  20:10                 0.5
etc. (times given on a 24 hour clock)

and goes on for years.  I have some code:

data<-read.table("H:/rainfall_data.txt",h=T)
library(survival)
datetime <- as.POSIXct(strptime(paste(data$V1, data$V2), "%d/%m/%Y
%H:%M"), tz="NZST")

which produces:

[10] "2003-03-13 21:13:00 New Zealand Daylight Time"
[11] "2003-03-15 13:20:00 New Zealand Daylight Time"
[12] "2003-03-15 22:20:00 New Zealand Daylight Time"
[13] "2003-03-15 22:21:00 New Zealand Daylight Time"
[14] "2003-03-16 00:02:00 New Zealand Daylight Time"
[15] "2003-03-16 18:10:00 New Zealand Standard Time"
[16] "2003-03-16 18:20:00 New Zealand Standard Time"
[17] "2003-03-16 18:30:00 New Zealand Standard Time"

My problem is that "15/03/2003 12:02" has become "16/03/2003 00:02"
i.e.  it is 12 hours behind (as is everything else), but also, I do not
want to change time zones.

The 12 hour delay is not really a problem just an annoyance, but the
time zone change is a problem because later on I need to match up data
by time using

mindata<-seq(from=min(datetime),to=max(datetime),by="mins")
newdata<-matrix(0,length(mindata),1)
newdata[match(format.POSIXct(datetime,"%Y %m %d %H
%M"),format.POSIXct(mindata,"%Y %m %d %H %M"))]<-data$V3

and things go wrong here with matching repeating times/missing times
around the timezone changes and, my resulting vector is 1 hour shorter
than my other series.  From the R help I see that my OS may be to blame
but, even if I specify tz="GMT" I still get NZST and NZDT.  Can someone
help?

I hope this all makes sense

Carla

-- 
Rich FitzJohn
rich.fitzjohn <at> gmail.com   |    http://homepages.paradise.net.nz/richa183
                     You are in a maze of twisty little functions, all alike