(define (now) (call-with-input-file "/proc/uptime" read))

I think it is all you need and that you should use it for all systems

(define (timeit thunk)
  (let ((start (now)))
    (thunk)
    (let ((end (now)))
      (- end start))))

(timeit (lambda () (wait-interval 1000)))
;Value: 1.

(timeit (lambda () (wait-interval 2000)))
;Value: 2.