Display

rednet.open("bottom")
m = peripheral.wrap("top")
m.clear()
m.setTextScale(1.5)

width, height = m.getSize()

departingTime = 0
untilDecrement = 0

timer = os.startTimer(0.05)

while true do
  event = {os.pullEvent()}

  if event[1] == "timer" then
    m.clear()

    if departingTime > 0 then
      if untilDecrement <= 0 then
        departingTime = departingTime - 1
        untilDecrement = 20
      else
        untilDecrement = untilDecrement - 1
      end
    end

    displayText = "Departing in:"
    m.setCursorPos(math.floor((width-#displayText)/2), (height/2)-1)
    m.write(displayText)

    displayText = departingTime.." seconds"
    m.setCursorPos(math.ceil((width-#displayText)/2), (height/2)+1)
    m.write(displayText)

    timer = os.startTimer(0.05)
  elseif event[1] == "rednet_message" then
    departingTime = tonumber(event[3]) or 0
    untilDecrement = 20
  end
end