[R] changing date by +/- days

[chuck.01]

Hi, 
Let say I have a date variable like this:

structure(1243792800, class = c("POSIXct", "POSIXt"), tzone = "GMT")
[1] "2009-05-31 18:00:00 GMT"

How can I make it 10 days prior, so:
[1] "2009-05-21 18:00:00 GMT"

I'm randomly selecting dates from a list  and want a  second value a set #
of days either before or after. 

Thanks ahead of time. 



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