http://blog.ixti.ru/?p=374

jquery.datepick.ext-maxRange.js

/*!
 * maxRange extension for $.datepick of Keith Wood (kbwood{at}iinet.com.au)
 * http://keith-wood.name/datepick.html
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * License: GNU/GPLv3 or higher (http://www.gnu.org/lisenses/gpl.txt)
 *
 * Copyright (c) 2010 Aleksey V. Zapparov (ixti{at}member.fsf.org)
 * http://www.ixti.ru
 *
 * Date: Mon May 17 21:45:01 CEST 2010
 */
;(function($) {

$.extend($.datepick, {
    limitMaxRange: function(dates) {
        var inst = $.data(this, $.datepick.dataName),
            days = (inst.settings.maxRange || 0) - 1,
            max  = null;

        //  skip if maxRange was not specified or was less or equal 0
        if (0 > days) { return; }

        // first run. preserve default maxDate
        if (undefined === inst.defaultMaxDate) {
            inst.defaultMaxDate = inst.get('maxDate');
        }

        // reset maxDate with datepicker on cleaning
        if (0 === dates.length) {
            $(this).datepick('option', 'maxDate', inst.defaultMaxDate);
            return;
        }

        // calculate maxDate if range picker was initiated
        if (inst.pickingRange) {
            max = new Date(dates[0].getTime() + (days*24*3600000));
            // dissallow new maxDate overlap default if it exists
            if ((null !== inst.defaultMaxDate)
                && (0 < (max.getTime() - inst.defaultMaxDate.getTime()))) {
                max = null;
            }
        }

        inst.settings.maxDate = max || inst.defaultMaxDate;
    }
});

})(jQuery);