I think this can be right for you.
aa <- 1433848856453
as.POSIXct(aa/1000, origin="1970-01-01")
[1] "2015-06-09 13:20:56.453 CEST"
Edit
Now I've realized that my code is not reproducible, because of my personal settings, but this can fix it and let you achieve the goal.
options(digits.secs=6)