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rtimezonedstlubridate

R: lubridate's dst() not working as expected

I suspect I am doing something very silly, but I can't get the dst function in lubridate to work as expected. 

library(lubridate)

x <- c("2016-01-01", "2016-06-01")

dst(x) # Returns c(FALSE, TRUE)
dst(as.Date(x)) # Returns c(FALSE, FALSE)

The result I expect in both cases is c(FALSE, TRUE). However, I only get the expected result if I pass dst a character vector and not with a Date object. I'm using OS X, my current timezone is PST (America/Los_Angeles).

Solution

  • dst() calls a piece of code which is essentially:

    c(NA, FALSE, TRUE)[as.POSIXlt(x)$isdst + 2]

    as.POSIXlt is by default:

    as.POSIXlt(x=, tz="")

    ...which will take your system timezone by default. So, given your location in L.A., let's look at:

    as.POSIXlt(x, tz="America/Los_Angeles")
#[1] "2016-01-01 PST" "2016-06-01 PDT"
c(NA, FALSE, TRUE)[as.POSIXlt(x, tz="America/Los_Angeles")$isdst + 2]
#[1] FALSE  TRUE

    Everything is fine. Hooray. Now, let's try with as.Date(x)

    as.POSIXlt(as.Date(x))
#[1] "2016-01-01 UTC" "2016-06-01 UTC"
as.POSIXlt(as.Date(x), tz="America/Los_Angeles")
#[1] "2016-01-01 UTC" "2016-06-01 UTC"

    Ohhh. So, as.POSIXlt does not play nicely with Date objects, and always returns UTC instead of the local timezone, and seemingly ignores any tz= argument. And since UTC does not abide by any daylight savings, you will always end up with FALSE returned.

    Looking at the R source code, this seems to be the case. In https://svn.r-project.org/R/trunk/src/main/datetime.c you can find:

    # R call:
#> as.POSIXlt.Date
#function (x, ...) 
#.Internal(Date2POSIXlt(x))

# source code:
#SEXP attribute_hidden do_D2POSIXlt(SEXP call, SEXP op, SEXP args, SEXP env)
#{
#...
setAttrib(ans, s_tzone, mkString("UTC"));

    ...as a hard-coded string.