Insert Todays Date Plus Ten Days

[Esben Stien]

I'm using this to insert todays date: 

;;insert current date in ISO format,timestamp
  (defun insert-date (prefix)
    "Insert the current date. With prefix-argument, use ISO format. With
   two prefix arguments, write out the day and month name."
    (interactive "P")
    (let ((format (cond
     ;;              ((not prefix) "%d.%m.%Y")
                   ((not prefix) "%Y-%m-%d")
                   ((equal prefix '(4)) "%Y-%m-%d")
                   ((equal prefix '(16)) "%A, %d. %B %Y")))
          (system-time-locale "nb_NO"))
      (insert (format-time-string format))))

, but I'm unsure how I can insert the date in 10 days. 

There doesn't seem to be a way to do arithmetic on
format-time-string?;).

Any pointers as to how I can do this?. 

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