Redstone timer code

local sides = require("sides")
local event = require("event")

local allowAnySide = true
local startSide = sides.north
local stopSide = sides.south

local startTime = nil

function timeFormat(seconds)
    seconds = tonumber(seconds)

    if seconds <= 0 then
        return "00:00:00";
    else
        hours = string.format("%02.f", math.floor(seconds/3600));
        mins = string.format("%02.f", math.floor(seconds/60 - (hours*60)));
        secs = string.format("%02.f", math.floor(seconds - hours*3600 - mins *60));
        return hours..":"..mins..":"..secs
    end
end

local function onStart()
    if startTime == nil then
        startTime = computer.uptime()
        print("Timer is running...")
    end
end

local function onStop()
    local time = computer.uptime()
    if startTime ~= nil then
        local diff = time - startTime
        print("Stop! Duration: " .. timeFormat(diff))
        startTime = nil
    end
end

local function onRedstone(e, address, side, oldValue, newValue)
    if oldValue == 0 then
        local signal = false
        if (allowAnySide == true and startTime == nil) or side == startSide then
            onStart()
            signal = true
        else
            if (allowAnySide == true and startTime ~= nil) or side == stopSide then
                onStop()
                signal = true
            end
        end
        if signal == false then
            if sides[side] ~= nil then
                side = sides[side]
            end
            print("Received redstone signal on side " .. side .. " matching no inputs.")
        end
    end
end

local run = true
local function onInterrupt()
    run = false
end

event.listen("redstone_changed", onRedstone)
event.listen("interrupted", onInterrupt)

print("Program started, waiting for input...")

while run do
    os.sleep(1)
end

event.ignore("redstone_changed", onRedstone)
event.ignore("interrupted", onInterrupt)

print("Program stopped")