The source code

    // generates a renderer function to be used for textual date groups
    Ext.util.Format.createTextDateRenderer = function(){
        // create the cache of ranges to be reused
        var today = new Date().clearTime(true);
        var year = today.getFullYear();
        var todayTime = today.getTime();
        var yesterday = today.add('d', -1).getTime();
        var tomorrow = today.add('d', 1).getTime();
        var weekDays = today.add('d', 6).getTime();
        var lastWeekDays = today.add('d', -6).getTime();

        var weekAgo1 = today.add('d', -13).getTime();
        var weekAgo2 = today.add('d', -20).getTime();
        var weekAgo3 = today.add('d', -27).getTime();

        var f = function(date){
            if(!date) {
                return '(No Date)';
            }
            var notime = date.clearTime(true).getTime();

            if (notime == todayTime) {
                return 'Today';
            }
            if(notime > todayTime){
                if (notime == tomorrow) {
                    return 'Tomorrow';
                }
                if (notime <= weekDays) {
                    return date.format('l');
                }
            }else {
            	if(notime == yesterday) {
                	return 'Yesterday';
                }
                if(notime >= lastWeekDays) {
                    return 'Last ' + date.format('l');
                }
            }
            return date.getFullYear() == year ? date.format('D m/d') : date.format('D m/d/Y');
       };

       f.date = today;
       return f;
    };