local function difftime(t1, t2)
    return os.time(t1) - os.time(t2) + (t1.mcs - t2.mcs) / 1000000
  end

  local n = 0
  local sum = 0
  local max = 0
  local min
  while run do
    local t1 = os.sysdate()
    local t2 = os.sysdate()
    while t1.mcs == t2.mcs do t2 = os.sysdate() end
    local dt = difftime(t2, t1) * 1000
    --message(string.format("%.3f ms", dt))
    n = n + 1
    sum = sum + dt
    if dt > max then max = dt end
    if min == nil or dt < min then min = dt end
    sleep(1)
  end
  message(string.format("avg: %.3f;  max: %.3f;  min: %.3f", sum / n, max, min))

avg: 1.049;  max: 6.480;  min: 0.067

  for i = 1, 10 do
    local t
    local t0 = os.time()
    repeat t = os.time() until t0 ~= t
    local s = os.sysdate()
    message(string.format("%.3f ms", 1000 * (t - os.time(s)) - s.mcs / 1000))
  end