Dynamic Analytics Bookmarks to load comparison date ranges year over year. This example only works as a comparison for the current date, versus the same day of the week last year.

bookmark-today.js

javascript:function url() {
    var start_date = new Date();
    var start_dd = start_date.getDate();
    var start_mm = start_date.getMonth() + 1;
    var start_yyyy = start_date.getFullYear();
    if (start_dd < 10) { start_dd = '0' + start_dd; }
    if (start_mm < 10) { start_mm = '0' + start_mm; }
    var start = '' + start_yyyy + start_mm + start_dd;
    
    var end_date = new Date();
    end_date.setMilliseconds(end_date.getMilliseconds() - 604800000 * 52);
    var end_dd = end_date.getDate();
    var end_mm = end_date.getMonth() + 1;
    var end_yyyy = end_date.getFullYear();
    if (end_dd < 10) { end_dd = '0' + end_dd; }
    if (end_mm < 10) { end_mm = '0' + end_mm; }
    var end = '' + end_yyyy + end_mm + end_dd;
    
    return 'https://analytics.google.com/analytics/web/#my-reports/**REPORTID*/%3F_u.date00%3D' + start + '%26_u.date01%3D' + start + '%26_u.date10%3D' + end + '%26_u.date11%3D' + end + '/'; 
};

window.open(url(), "_self");

NOTE: you must remove all line breaks and comments from this code before pasting it into a Chrome bookmark.