Javascript Pretty Date formatter

PrettyDate.js

/*
Pretty Date script modified from John Resig here http://ejohn.org/blog/javascript-pretty-date
*/
    function relativeDateTime (tdate) {
        var system_date;
        if (typeof (tdate) === "number") {
            tdate = new Date(tdate).toString();
        }

        if (navigator.userAgent.match(/MSIE\s([^;]*)/)) {
            system_date = Date.parse(tdate.replace(/( \+)/, ' UTC$1'))
        } else {
            system_date = new Date(Date.parse(tdate));
        }

        var user_date = new Date();

        var diff = Math.floor((user_date - system_date) / 1000);
        if (diff <= 1) { return "just now"; }
        if (diff < 20) { return diff + " seconds ago"; }
        if (diff < 40) { return "half a minute ago"; }
        if (diff < 60) { return "less than a minute ago"; }
        if (diff <= 90) { return "one minute ago"; }
        if (diff <= 3540) { return Math.round(diff / 60) + " minutes ago"; }
        if (diff <= 5400) { return "1 hour ago"; }
        if (diff <= 86400) { return Math.round(diff / 3600) + " hours ago"; }
        if (diff <= 129600) { return "1 day ago"; }
        if (diff < 604800) { return Math.round(diff / 86400) + " days ago"; }
        if (diff <= 777600) { return "1 week ago"; }
        // Clock and calendar identifiers 
        var calendarIdentifiers = Windows.Globalization.CalendarIdentifiers; 
        var clockIdentifiers = Windows.Globalization.ClockIdentifiers;  
        var formatter = new Windows.Globalization.DateTimeFormatting.DateTimeFormatter("longdate",
            ["en-US"], "US", calendarIdentifiers.gregorian, clockIdentifiers.twentyFourHour);
        return "on " + formatter.format(new Date(tdate));
    }

    function parseDate (timeString) {
        var date = Date.parse((timeString || "").replace(/-/g, "/").replace(/TZ/g, " ").replace(/( \+)/, ' UTC$1'));
        return date;
    }