Timer (ticks only while the chunk is loaded)

local monitorSide = "left"
local refreshInterval = 5 --seconds

function secondsToHMS(seconds, leadingZero)
    local h = math.floor(seconds / 3600)
    local m = math.floor(seconds % 3600 / 60)
    local s = seconds % 60
    if leadingZero then
        h = tostring(h)
        m = tostring(m)
        s = tostring(s)
        if string.len(h) < 2 then
            h = "0"..h
        end
        if string.len(m) < 2 then
            m = "0"..m
        end
        if string.len(s) < 2 then
            s = "0"..s
        end
    end
    return h, m, s
end

function getIngameEpochAlternative()
    --ingame days in hours plus ingame day hours converted ( * 3600 / 72 ) to real-world millis
    return ( os.time("ingame") + os.day("ingame") * 24 ) * 50 * 1000
end

term.clear()
term.setCursorPos(1,1)
local saveData = {}
local utcStarted = os.epoch("utc")
local ingameStarted = getIngameEpochAlternative()
saveData["utcResumed"] = 0
saveData["ingameResumed"] = 0
local file = fs.open("timer.save","r")
if file ~= nil then
    local data = file.readAll()
    file.close()
    saveData = textutils.unserialize(data)
    local h, m, s = secondsToHMS(math.floor(saveData["utcResumed"]/1000), true)
    print(h .. ":" .. m .. ":" .. s .. " since the timer was started, resuming...")
else
    file = fs.open("timer.save","w")
    file.write(textutils.serialize(saveData))
    file.close()
end
local utcResumed = saveData["utcResumed"]
local ingameResumed = saveData["ingameResumed"]
term.setCursorPos(1,3)
term.write("UTC:")
term.setCursorPos(1,5)
term.write("ingame:")
term.setCursorPos(1,17)
term.write("[R] - Reset")

local monitor = peripheral.wrap(monitorSide)
if monitor~=nil then
    monitor.setTextColor(colors.black)
    monitor.setBackgroundColor(colors.orange)
end

while true do
    --print time(s)
    local utc = os.epoch("utc") - utcStarted + utcResumed
    local ingame = getIngameEpochAlternative() - ingameStarted + ingameResumed

    term.setCursorPos(1,4)
    local h, m, s = secondsToHMS(math.floor(utc/1000), true)
    term.write(h .. ":" .. m .. ":" .. s)

    term.setCursorPos(1,6)
    local h, m, s = secondsToHMS(math.floor(ingame/1000), true)
    term.write(h .. ":" .. m .. ":" .. s)

    if monitor~=nil then
        monitor.clear()
        monitor.setTextScale(5)
        monitor.setCursorPos(1,1)
        monitor.write(h .. ":" .. m)
    end

    --update saveData
    saveData["utcResumed"] = utc
    saveData["ingameResumed"] = ingame
    file = fs.open("timer.save","w")
    file.write(textutils.serialize(saveData))
    file.close()

    --start refresh timer
    local timer = os.startTimer(refreshInterval)
    local event, b, c, d = os.pullEvent()
    os.cancelTimer(timer)

    --listen for events (timer, key press, ...)
    if event == "key" then
        if keys.getName(b) == "r" then
            utcStarted = os.epoch("utc")
            ingameStarted = getIngameEpochAlternative()
            utcResumed = 0
            ingameResumed = 0
            term.setCursorPos(1,1)
            term.clearLine()
        end
    end
end