set timeBack to (-1 * weeks)
tell application "iCal"
activate
repeat with allCals in calendars
repeat with myEvent in events of allCals
set (start date of myEvent) to ((start date of myEvent) + (my timeBack))
set (end date of myEvent) to ((end date of myEvent) + (my timeBack))
end repeat
repeat with myTodo in todos of allCals
if (due date of myTodo exists) and not (completion date of myTodo exists) then
set due date of myTodo to (due date of myTodo) + timeBack
end if
end repeat
end repeat
end tell