h = 2
m = 49
s = 60

function check()

if s <= 0 then
  m = m -1
  s = 60
end

if m < 0 then
  h = h-1
  m = 60
end

if h < 0 then
  redstone.setOutput("back", true)
  sleep(1)
  redstone.setOutput("back", false)
  sleep(300)
  h = 02
  m = 49
  s = 60
end
end
while true do
term.clear()
term.setCursorPos(1,1)
print(h..":"..m..":"..s)
sleep(1)
s = s-1
check()
end