x<-as.POSIXct(1378182603, origin="1970-01-01")
strftime(x, format= "%Y-%m-%d %H:%M:%S")
#[1] "2013-09-03 05:30:03"
It is processed as though it is recorded in UCT but output as if in the America/New_York timezone.
x<-as.POSIXct(1378182603, origin="1970-01-01")
strftime(x, format= "%Y-%m-%d %H:%M:%S", tz="America/Los_Angeles")
#[1] "2013-09-03 02:30:03"