[R] ts vs. POSIX

[Jason Turner]

Erin Hodgess wrote:
> What if I have a time series which is collected every Monday, please?
> 
> What is the proper way to use the start option within the ts command
> in order to indicate that this is Monday data, please?
> 

ts objects don't directly support dates.  There is some provision for 
monthly data, but this isn't the same as uniform, across-the-board date 
support.  What they do have is a start time, a deltat, and frequency 
(observations per period).  The main reason to use ts objects *isn't* 
the date/time handling, but for the nice functions (acf, spectrum, etc) 
you can use for regularly spaces time samples.

For weekly data, I'd use one of the following approaches (assuming the 
series starts in the first Monday of 2003):

1)
# dates in a year,week format
 > foo <- ts(1:100,start=c(2003,1),frequency=52)

or

2)
# dates as numeric representation of POSIXct objects
foo <- ts(1:100,start=as.numeric(as.POSIXct("2003-1-6")),deltat=60*60*24*7)
 > start(foo)
[1] 1041764400
 > end(foo)
[1] 1101639600
 > last <- end(foo)
 > class(last) <- "POSIXct"
 > last
[1] "2004-11-29 New Zealand Daylight Time"

(2) depends on as.numeric(POSIXct.object) giving a sensible, 
single-digit answer.  This is not guaranteed.  It works today, but 
nobody promised this approach would work tomorrow.

Hope that helps.

Cheers

Jason
-- 
Indigo Industrial Controls Ltd.
http://www.indigoindustrial.co.nz
64-21-343-545
jasont at indigoindustrial.co.nz