Tweaked utm-alternative that adds each visit to the cookie. Automatically removes referrals from your own site. Skips addition if new hit is same as previous. Original here: https://github.com/dm-guy/utm-alternative

traffic-source 1.1.js

(function(cookieName, domain){

    var traffic_source_COOKIE_TOKEN_SEPARATOR = ">>"; //separating between concatenated lead source
    var traffic_source_meta_SEPARATOR = ">";
    var traffic_source_date_SEPARATOR = "|";
    var NONE = "(none)";

    domain = domain || window.location.hostname;

    function getCookie(cookieName){
        var name = cookieName + "=";
        var cookieArray = document.cookie.split(';'); //break cookie into array
        for(var i = 0; i < cookieArray.length; i++){
          var cookie = cookieArray[i].replace(/^\s+|\s+$/g, ''); //replace all space with '' = delete it
          if (cookie.indexOf(name)==0){
             return cookie.substring(name.length,cookie.length); //
          }
        }
        return null;
    }

/*
*  Takes the current time of the session, and optional addition to the traffic source. 
*  The output will be of the format: >>{traffic source}|>DD-MM-YY HH:MM AM/PM>>
*
*/

/*
*   Uncomment currTime() in order to activate. You should also uncomment the lines in the code that call this function (all marked with a comment). If you do so, uncomment the line above them which does not employ currTime()


	function currTime() {
	    var date = new Date();
	    var currDate = date.getDate();
	    var hours = date.getHours();
	    var minutes = date.getMinutes();
	    var month = date.getMonth() + 1;
	    var year = date.getFullYear();
	    var ampm = hours >= 12 ? 'pm' : 'am';
	    hours = hours % 12;
	    hours = hours ? hours : 12; // the hour '0' makes '12'
	    minutes = minutes < 10 ? '0' + minutes : minutes;
	    var strTime = traffic_source_date_SEPARATOR + currDate + '-' + month + '-' + year + ' ' + hours + ':' + minutes + ' ' + ampm + ')';
	    return strTime;
	}

*/

	/*
       Checks whether a certain parameter exist in the current browser URL. If it does, it returns its name. 
	   It will receive "src" later in the main function
    */
	
    function getURLParameter(param){
        var pageURL = window.location.search.substring(1); //get the query string parameters without the "?"
        var URLVariables = pageURL.split('&'); //break the parameters and values attached together to an array
        for (var i = 0; i < URLVariables.length; i++) {
            var parameterName = URLVariables[i].split('='); //break the parameters from the values
            if (parameterName[0] == param) {
                return parameterName[1];
            }
        }
        return null;
    }

	/*
       Gets the first (latest) token from a cookie: THIS__ONE>>NOT__THIS__ONE>>AND__NOT__THIS__ONE
    */
	
    function getFirstTokenFromCookie(cookie){
        var result = "";
        var firstSeparatorIndex = cookie.indexOf(traffic_source_COOKIE_TOKEN_SEPARATOR);
        result = firstSeparatorIndex !== -1 ? cookie.substring(0, firstSeparatorIndex) : cookie; //if there is a separator, provide the newest value no the cookie  
        return result;
    }

	/*
       Set the cookie if it doesn't exist.
    */
	
    function setCookie(cookie, value){
        var expires = new Date();
        expires.setTime(expires.getTime() + 62208000000); //1000*60*60*24*30*24 (2 years)
        document.cookie = cookie + "=" + value + "; expires=" + expires.toGMTString() + "; domain=" + domain + "; path=/";
    }
	
	/*
       Boolean, whether or not it's not NULL or Empty
    */
	
    function isNotNullOrEmpty(string){
        return string !== null && string !== "";
    }
	
	/*
       Remove the protocol for the referral token
    */
    function removeProtocol(href) {
        return href.replace(/.*?:\/\//g, "");
    }

    /*
    Add the UMTZ cookie as base for the cookie
    */
    function setUMTZdefault(){
        var utmzCookie = getCookie("__utmz"); //get GA cookie
        var cookieCampaignName = "";  //reset lead source value
        if(utmzCookie != null) {
            var UTMSRC = "utmccn=";
            var start = utmzCookie.indexOf(UTMSRC);
            var end = utmzCookie.indexOf("|", start); 
            if(start > -1) {
                if(end === -1) {
                    end = utmzCookie.length; 
                }
                cookieCampaignName = "utmz" + traffic_source_meta_SEPARATOR + utmzCookie.substring((start + UTMSRC.length), end); //get the value of the UTMZ, without the parameter name
                setCookie(cookieName, cookieCampaignName); //set the default cookie
            }
        }
    }

    function isRealReferrer(){
        return (document.referrer != "" && document.referrer.indexOf(location.protocol + "//" + location.host) !== 0);
    }

    /*
       Find lead source by checking cookie, url param "src" or referrer(that order) and set new cookie traffic_source.
       Do this only if cookie do not exist. This value will be used for contact form.
       If cookie exist add url "src" param on the beginning of the cookie if it exists and it is not already at the beginning of the cookie.
       This way "tokens" are made in traffic_source cookie. Token separator is ">>" (see above traffic_source_COOKIE_TOKEN_SEPARATOR)
    */
    
    var traffic_source = ""; //reset lead source value

    if(document.cookie.indexOf(cookieName) === -1) { // if there is no cookie traffic_source set yet, check if there is an existing UTMZ campaign name.
        setUMTZdefault();
    }

    var urlParamSRC = getURLParameter("src"); //take the SRC value

    if(isNotNullOrEmpty(urlParamSRC)) { //if there is not campaign name but we have SRC
        traffic_source = decodeURI(urlParamSRC); // decode URI of the SRC parameter
        // traffic_source = decodeURI(urlParamSRC) + currTime(); //  Optional: add the session time
    }
    else if(isRealReferrer()){ //if there is not campaign, and no src, check if there is a referrer we want to keep
        var referrerHostName = removeProtocol(document.referrer);
        var GOOGLE = /www.google/; 
        var YAHOO = /search.yahoo/; 
        var BING = /www.bing/; 
        if(GOOGLE.test(referrerHostName)){
            traffic_source = "Google Search";
            //traffic_source = "Google Search" + currTime(); // + Optional: Add the current session time. Choose one of them.
        } else if(YAHOO.test(referrerHostName)){
            traffic_source = "Yahoo Search";
            //traffic_source = "Yahoo Search" + currTime();
        } else if(BING.test(referrerHostName)){
            traffic_source = "Bing Search";
            //traffic_source = "Bing Search" + currTime();
        } else {
            traffic_source = referrerHostName;
            //traffic_source = referrerHostName + currTime();
        }
        traffic_source = "referral" + traffic_source_meta_SEPARATOR + traffic_source;
    } else if (document.referrer == "") {
        traffic_source = NONE;
    }

    var firstToken = getFirstTokenFromCookie(getCookie(cookieName));
    if(traffic_source != "" && traffic_source !== firstToken) {
        var newTrafficSourceCookie = traffic_source + traffic_source_COOKIE_TOKEN_SEPARATOR + getCookie(cookieName); //concetenate new SRC with old cookie values
        // var newTrafficSourceCookie = urlParamSRC + currTime() + traffic_source_COOKIE_TOKEN_SEPARATOR + getCookie(cookieName); // Optional: add the time of current session
        setCookie(cookieName, newTrafficSourceCookie);
    }

 })("traffic_source", ".YOURDOMAIN.COM");

I am getting Uncaught TypeError: Cannot read property 'indexOf' of null when using this. Most probably when the cookie is not yet set (user is visiting for the first time) cookie is null in this line of code -> cookie.indexOf(traffic_source_COOKIE_TOKEN_SEPARATOR);