Convert it to the zooreg class (so that it knows that it is regularly posted or almost like that), and then leave it appropriately:
> lag(as.zooreg(tail(aapl)), -7) 2013-05-14 2013-05-15 2013-05-16 2013-05-17 2013-05-20 2013-05-21 455.64 460.79 456.77 452.97 454.74 443.86
For more information, check out the ?zooreg examples section and read the comments, starting from where it reads:
# lag and diff (as diff is defined in terms of lag)
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