if #arg < 1 then
  print("usage: clock [side of monitor]")
  os.exit()
end

monitor = peripheral.wrap(arg[1])

function drawTime()
  time = os.time()
  timeText = textutils.formatTime(time, true)

  -- horizontally center
  w, h = monitor.getSize()
  x = 1 + (w - 5) / 2 + (5 - string.len(timeText))
  y = 1 + (h - 1) / 2

  -- draw
  monitor.clear()
  monitor.setCursorPos(x, y)
  monitor.write(timeText)
end

while true do
  drawTime()
  os.sleep(1)
end