The POSIXct variable xx has a tzone attribute but if I add 1 second to it
then the tzone attribute gets stripped.  Is that intended?

> xx <- as.POSIXct(format(Sys.time()), tz = "")
> dput(xx)
structure(1259541335.099, class = c("POSIXt", "POSIXct"), tzone = "")
> dput(xx+1)
structure(1259541336.099, class = c("POSIXt", "POSIXct"))

> R.version.string # Windows Vista
[1] "R version 2.10.0 Patched (2009-11-21 r50532)"

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