Timer.js

// Class: Timer
var Timer = function (callback) {
    // Property: Frequency of elapse event of the timer in milliseconds
    this.Interval = 1000;

    // Property: Whether the timer is enable or not
    this.Enable = new Boolean(false);

    // Event: Timer tick
    this.Tick = callback;

    // Member variable: Hold interval id of the timer
    var timerId = 0;

    // Member variable: Hold instance of this class
    var thisObject;

    // Function: Start the timer
    this.Start = function () {
        this.Enable = new Boolean(true);

        thisObject = this;
        if (thisObject.Enable) {
            thisObject.timerId = setInterval(
            function () {
                thisObject.Tick();
            }, thisObject.Interval);
        }
    };

    // Function: Stops the timer
    this.Stop = function () {
        thisObject.Enable = new Boolean(false);
        clearInterval(thisObject.timerId);
    };

};

// Namespace: Match rules and timings
var Match = {

    Timers: {
        FirstHalf: new Timer(TimerTick),
        HalfTime: new Timer(TimerTick),
        SecondHalf: new Timer(TimerTick),
        TickCount: -1
    },

    Strings: {
        FirstHalf: 'First Half',
        HalfTime: 'Half Time',
        SecondHalf: 'Second Half',
        FullTime: 'Finished'
    },

    DisplayTime: function (t) {
        var m = parseInt(t / 60);
        var s = t % 60;
        return (m < 10 ? '0' + m : m) + ":" + (s < 10 ? '0' + s : s);
    }
};

// Function: Tick Event Handler (callback function)
function TimerTick(timer) {

    // Document elements used.
    var TimerP = document.getElementById('time');
    var DisplayP = document.getElementById('display');

    // During First Half
    if (Match.Timers.FirstHalf.Enable == true) {
        if (Match.Timers.TickCount == -1) { Match.Timers.TickCount = 0 }
        if (Match.Timers.TickCount == 2700) {
            Match.Timers.FirstHalf.Stop();
            Match.Timers.TickCount = -1;
            Match.Timers.HalfTime.Start();
        } else {
            TimerP.innerHTML = Match.DisplayTime(Match.Timers.TickCount);
            DisplayP.innerHTML = Match.Strings.FirstHalf;
            Match.Timers.TickCount++;
        }
    }

    // During Half Time
    else if (Match.Timers.HalfTime.Enable == true) {
        if (Match.Timers.TickCount == -1) { Match.Timers.TickCount = 0 }
        if (Match.Timers.TickCount == 900) {
            Match.Timers.HalfTime.Stop();
            Match.Timers.TickCount = -1;
            Match.Timers.SecondHalf.Start();
        } else {
            TimerP.innerHTML = '45:00';
            DisplayP.innerHTML = Match.Strings.HalfTime + ' (' + Match.DisplayTime(900 - Match.Timers.TickCount) + ')';
            Match.Timers.TickCount++;
        }
    }

    // During Second Half
    else if (Match.Timers.SecondHalf.Enable == true) {
        if (Match.Timers.TickCount == -1) { Match.Timers.TickCount = 2700 }
        if (Match.Timers.TickCount == 5400) {
            TimerP.innerHTML = '90:00';
            DisplayP.innerHTML = Match.Strings.FullTime;
            Match.Timers.SecondHalf.Stop();
            Match.Timers.TickCount = -1;
        } else {
            TimerP.innerHTML = Match.DisplayTime(Match.Timers.TickCount);
            DisplayP.innerHTML = Match.Strings.SecondHalf;
            Match.Timers.TickCount++;
        }
    }
}

function KickOff() {
    var btn = document.getElementById('btnKickOff');
    btn.setAttribute('style','display: none;');
    Match.Timers.FirstHalf.Start();
}