converting #sec to POSIX* format

Sure I'm missing something simple here, but I haven't been able to find a
easy way to convert a pure numeric (# of seconds since the beginning of
1970) back into a POSIX* object. 

ex.

z<-as.POSIXct(Sys.time()) 
z

[1] "2002-05-01 10:33:56 Mountain Daylight Time"

x<-unclass(z)
x

[1] 1020270836

Is there a simple way to transform x back into a POSIX* object similar to z?

Landon Jensen
Micron Technology, Inc.
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