Converting date times in POSIXct gives screwy result?

Question

I'm doing some extracting of data from a database, and running the results through rehsape2. For some reason this is mangling the POSIXct datetime stamps into numeric. No Problem I think, you can just turn them back, except I'm an hour out.

Here's a minimal example

foo<-as.POSIXct("2011-04-04 14:18:58")
as.numeric(foo)     #gives 130192318
bar<-as.POSIXct(as.numeric(foo), 
                tz=Sys.timezone(),
                origin=as.POSIXct(
                  strptime("1970-01-01 00:00:00", "%Y-%m-%d %H:%M:%S", tz="UTC")))
as.numeric(bar)     #gives 130192318 identical !
foo     #Gives "2011-04-04 14:18:58 BST"
bar     #Gives "2011-04-04 13:18:58 UTC"

Obviously foo and bar are numerically identical, but R thinks foo needs to be displayed as BST and bar as UTC. How do I get both displayed as BST. This doesn't work either;

as.POSIXct(bar, tz="BST")   #still gives "2011-04-04 13:18:58 UTC"

Answer 1

Here's what's going on. bar is created using as.POSIXct.numeric, which is defined as:

as.POSIXct.numeric
function (x, tz = "", origin, ...) 
{
    if (missing(origin)) 
        stop("'origin' must be supplied")
    as.POSIXct(origin, tz = tz, ...) + x
}
<environment: namespace:base>

You supply an origin that is a POSIXct object. That means the as.POSIXct call in as.POSIXct.numeric dispatches to as.POSIXct.default, which is defined as:

as.POSIXct.default
function (x, tz = "", ...) 
{
    if (inherits(x, "POSIXct")) 
        return(x)
    if (is.character(x) || is.factor(x)) 
        return(as.POSIXct(as.POSIXlt(x, tz, ...), tz, ...))
    if (is.logical(x) && all(is.na(x))) 
        return(.POSIXct(as.numeric(x)))
    stop(gettextf("do not know how to convert '%s' to class \"POSIXct\"", 
        deparse(substitute(x))))
}
<environment: namespace:base>

x is a POSIXct class object (the origin you supplied in your initial call), so it is simply returned and the tz= argument is ignored.

---

UPDATE:
Here's how you can convert foo back to POSIXct with the appropriate time zone.

(foo <- as.POSIXct("2011-04-04 14:18:58", tz="GB"))
# [1] "2011-04-04 14:18:58 BST"
.POSIXct(as.numeric(foo), tz="GB")
# [1] "2011-04-04 14:18:58 BST"