Truncating dates (and other date-time manipulations)

Look at the ti (Time Index) class in package tis.  Here's some examples I just did:

x <- Sys.Date()
x

[1] "2008-09-11"

ti(x, "wsaturday")       ## a ti for the week that x falls into

[1] 20080913
class: ti

ti(x + 1, "wsaturday") - 1  ## ti for the latest complete week

[1] 20080906
class: ti

as.Date(ti(x + 1, "wsaturday") - 1)  ## as a Date

[1] "2008-09-06"

ymd(ti(x + 1, "wsaturday") - 1)  ## or just the yyyymmdd number

[1] 20080906

Hmmm:

dates <- structure(c(8516, 8544, 8568, 8596, 8609, 8666, 8701, 8750, 8754,
8798, 8811, 8817, 8860, 8873, 8918, 8931, 8966, 9020, 9034, 9056
), class = "Date")

range(dates)

[1] "1993-04-26" "1994-10-18"

as.Date(ti(range(dates), "monthly"))

[1] "1993-04-30" "1994-10-31"

as.Date(ti(range(dates), "annual"))

[1] "1993-12-31" "1994-12-31"

What's the easiest way to control whether the dates are rounded up or
down?  And to get the starting period I just subtract one from the ti
object and then add one to the Date object?

Is there any standard way to do multiples of periods?  (e.g. 3 weeks,
4 days).  And is the finest resolution of a ti object a day?  I don't
think I see any periods less than that in the output of tif()

Hadley

http://had.co.nz/