Try this!

timer = function (time)
    local init = os.time()
    local diff=os.difftime(os.time(),init)
    while diff<time do
        coroutine.yield(diff)
        diff=os.difftime(os.time(),init)
    end
    print( 'Timer timed out at '..time..' seconds!')
end
co=coroutine.create(timer)
coroutine.resume(co,30) -- timer starts here!
while coroutine.status(co)~="dead" do
    print("time passed",select(2,coroutine.resume(co)))
    print('',coroutine.status(co))
    os.sleep(5)
end