running = false
startTime = os.clock()
endTime = os.clock()
monitor = peripheral.wrap("left")
monitor.setTextScale(4)

function round(num, numDecimalPlaces)
  local mult = 10^(numDecimalPlaces or 0)
  return math.floor(num * mult + 0.5) / mult
end

while true do
  os.pullEvent("redstone")
  if rs.getInput("bottom") then
    running = true
    startTime = os.clock()
    print("Start: " .. startTime)
    monitor.setCursorPos(1,2)
    monitor.write(" ...")
  elseif rs.getInput("back") and running then
    running = false
    endTime = os.clock()
    print("End: " .. endTime)
    delta = round(endTime - startTime, 2)
    monitor.setCursorPos(1,2)
    monitor.write("    ")
    monitor.setCursorPos(1,2)
    monitor.write(delta)
    print(delta)
  end
end