POSIXct values become numeric in reshape2 dcast

Doing debug(dcast) and debug(as.data.frame.matrix), then stepping through the calculations launched by your dcast() call will reveal that these lines in as.data.frame.matrix() are at fault:

if (mode(x) == "character" && stringsAsFactors) {
    for (i in ic) value[[i]] <- as.factor(x[, i])
}
else {
    for (i in ic) value[[i]] <- as.vector(x[, i])
}

The up-to-then POSIXct object has mode "numeric", so evaluation follows the second branch, which converts the results to numeric.

If you use dcast(), it looks like you will need to post-process results, which shouldn't be too hard if you have the correct origin. Something like this (which doesn't quite get the origin right) should do the trick:

e[-1] <- lapply(e[-1], as.POSIXct, origin="1960-01-01")

---

FWIW, base R's reshape() leaves POSIXct values as they are but will require you to edit the names of the resulting columns...

reshape(d, idvar="x", timevar="y",  direction="wide")
#   x                 z.c                 z.d
# 1 a 2012-01-01 01:01:01                <NA>
# 2 b                <NA> 2012-02-02 02:02:02