redOn = false
while true do
    local ev,p1 = os.pullEvent("redstone")
    if ev == "redstone" then
        if rs.testBundledInput("back",colors.red) then
            if not redOn then
                redOn = true
                redStart = os.clock()
            end
        else
            if redOn then
                redOn = false
                timePassed  = os.clock() - redStart
                print("Seconds passed: " .. timePassed)
            end
        end
    end
end