The source code

    /**
     * @class Ext.DatePicker
     * @extends Ext.Component
     * Simple date picker class.
     * @constructor
     * Create a new DatePicker
     * @param {Object} config The config object
     * @xtype datepicker
     */
    Ext.DatePicker = Ext.extend(Ext.BoxComponent, {
        /**
         * @cfg {String} todayText
         * The text to display on the button that selects the current date (defaults to 'Today')
         */
        todayText : 'Today',
        /**
         * @cfg {String} okText
         * The text to display on the ok button (defaults to ' OK ' to give the user extra clicking room)
         */
        okText : ' OK ',
        /**
         * @cfg {String} cancelText
         * The text to display on the cancel button (defaults to 'Cancel')
         */
        cancelText : 'Cancel',
        /**
         * @cfg {Function} handler
         * Optional. A function that will handle the select event of this picker.
         * The handler is passed the following parameters:
         * picker : DatePickerThe Ext.DatePicker.
         * date : DateThe selected date.
         *
         */
        /**
         * @cfg {Object} scope
         * The scope (this reference) in which the {@link #handler}
         * function will be called.  Defaults to this DatePicker instance.
         */
        /**
         * @cfg {String} todayTip
         * The tooltip to display for the button that selects the current date (defaults to '{current date} (Spacebar)')
         */
        todayTip : '{0} (Spacebar)',
        /**
         * @cfg {String} minText
         * The error text to display if the minDate validation fails (defaults to 'This date is before the minimum date')
         */
        minText : 'This date is before the minimum date',
        /**
         * @cfg {String} maxText
         * The error text to display if the maxDate validation fails (defaults to 'This date is after the maximum date')
         */
        maxText : 'This date is after the maximum date',
        /**
         * @cfg {String} format
         * The default date format string which can be overriden for localization support.  The format must be
         * valid according to {@link Date#parseDate} (defaults to 'm/d/y').
         */
        format : 'm/d/y',
        /**
         * @cfg {String} disabledDaysText
         * The tooltip to display when the date falls on a disabled day (defaults to 'Disabled')
         */
        disabledDaysText : 'Disabled',
        /**
         * @cfg {String} disabledDatesText
         * The tooltip text to display when the date falls on a disabled date (defaults to 'Disabled')
         */
        disabledDatesText : 'Disabled',
        /**
         * @cfg {Array} monthNames
         * An array of textual month names which can be overriden for localization support (defaults to Date.monthNames)
         */
        monthNames : Date.monthNames,
        /**
         * @cfg {Array} dayNames
         * An array of textual day names which can be overriden for localization support (defaults to Date.dayNames)
         */
        dayNames : Date.dayNames,
        /**
         * @cfg {String} nextText
         * The next month navigation button tooltip (defaults to 'Next Month (Control+Right)')
         */
        nextText : 'Next Month (Control+Right)',
        /**
         * @cfg {String} prevText
         * The previous month navigation button tooltip (defaults to 'Previous Month (Control+Left)')
         */
        prevText : 'Previous Month (Control+Left)',
        /**
         * @cfg {String} monthYearText
         * The header month selector tooltip (defaults to 'Choose a month (Control+Up/Down to move years)')
         */
        monthYearText : 'Choose a month (Control+Up/Down to move years)',
        /**
         * @cfg {Number} startDay
         * Day index at which the week should begin, 0-based (defaults to 0, which is Sunday)
         */
        startDay : 0,
        /**
         * @cfg {Boolean} showToday
         * False to hide the footer area containing the Today button and disable the keyboard handler for spacebar
         * that selects the current date (defaults to true).
         */
        showToday : true,
        /**
         * @cfg {Date} minDate
         * Minimum allowable date (JavaScript date object, defaults to null)
         */
        /**
         * @cfg {Date} maxDate
         * Maximum allowable date (JavaScript date object, defaults to null)
         */
        /**
         * @cfg {Array} disabledDays
         * An array of days to disable, 0-based. For example, [0, 6] disables Sunday and Saturday (defaults to null).
         */
        /**
         * @cfg {RegExp} disabledDatesRE
         * JavaScript regular expression used to disable a pattern of dates (defaults to null).  The {@link #disabledDates}
         * config will generate this regex internally, but if you specify disabledDatesRE it will take precedence over the
         * disabledDates value.
         */
        /**
         * @cfg {Array} disabledDates
         * An array of 'dates' to disable, as strings. These strings will be used to build a dynamic regular
         * expression so they are very powerful. Some examples:
         *
         * ['03/08/2003', '09/16/2003'] would disable those exact dates
         * ['03/08', '09/16'] would disable those days for every year
         * ['^03/08'] would only match the beginning (useful if you are using short years)
         * ['03/../2006'] would disable every day in March 2006
         * ['^03'] would disable every day in every March
         *
         * Note that the format of the dates included in the array should exactly match the {@link #format} config.
         * In order to support regular expressions, if you are using a date format that has '.' in it, you will have to
         * escape the dot when restricting dates. For example: ['03\\.08\\.03'].
         */

        // private
        initComponent : function(){
            Ext.DatePicker.superclass.initComponent.call(this);

            this.value = this.value ?
                     this.value.clearTime() : new Date().clearTime();

            this.addEvents(
                /**
                 * @event select
                 * Fires when a date is selected
                 * @param {DatePicker} this
                 * @param {Date} date The selected date
                 */
                'select'
            );

            if(this.handler){
                this.on('select', this.handler,  this.scope || this);
            }

            this.initDisabledDays();
        },

        // private
        initDisabledDays : function(){
            if(!this.disabledDatesRE && this.disabledDates){
                var dd = this.disabledDates,
                    len = dd.length - 1,
                    re = '(?:';

                Ext.each(dd, function(d, i){
                    re += Ext.isDate(d) ? '^' + Ext.escapeRe(d.dateFormat(this.format)) + '$' : dd[i];
                    if(i != len){
                        re += '|';
                    }
                }, this);
                this.disabledDatesRE = new RegExp(re + ')');
            }
        },

        /**
         * Replaces any existing disabled dates with new values and refreshes the DatePicker.
         * @param {Array/RegExp} disabledDates An array of date strings (see the {@link #disabledDates} config
         * for details on supported values), or a JavaScript regular expression used to disable a pattern of dates.
         */
        setDisabledDates : function(dd){
            if(Ext.isArray(dd)){
                this.disabledDates = dd;
                this.disabledDatesRE = null;
            }else{
                this.disabledDatesRE = dd;
            }
            this.initDisabledDays();
            this.update(this.value, true);
        },

        /**
         * Replaces any existing disabled days (by index, 0-6) with new values and refreshes the DatePicker.
         * @param {Array} disabledDays An array of disabled day indexes. See the {@link #disabledDays} config
         * for details on supported values.
         */
        setDisabledDays : function(dd){
            this.disabledDays = dd;
            this.update(this.value, true);
        },

        /**
         * Replaces any existing {@link #minDate} with the new value and refreshes the DatePicker.
         * @param {Date} value The minimum date that can be selected
         */
        setMinDate : function(dt){
            this.minDate = dt;
            this.update(this.value, true);
        },

        /**
         * Replaces any existing {@link #maxDate} with the new value and refreshes the DatePicker.
         * @param {Date} value The maximum date that can be selected
         */
        setMaxDate : function(dt){
            this.maxDate = dt;
            this.update(this.value, true);
        },

        /**
         * Sets the value of the date field
         * @param {Date} value The date to set
         */
        setValue : function(value){
            var old = this.value;
            this.value = value.clearTime(true);
            if(this.el){
                this.update(this.value);
            }
        },

        /**
         * Gets the current selected value of the date field
         * @return {Date} The selected date
         */
        getValue : function(){
            return this.value;
        },

        // private
        focus : function(){
            if(this.el){
                this.update(this.activeDate);
            }
        },

        // private
        onEnable: function(initial){
            Ext.DatePicker.superclass.onEnable.call(this);
            this.doDisabled(false);
            this.update(initial ? this.value : this.activeDate);
            if(Ext.isIE){
                this.el.repaint();
            }

        },

        // private
        onDisable : function(){
            Ext.DatePicker.superclass.onDisable.call(this);
            this.doDisabled(true);
            if(Ext.isIE && !Ext.isIE8){
                /* Really strange problem in IE6/7, when disabled, have to explicitly
                 * repaint each of the nodes to get them to display correctly, simply
                 * calling repaint on the main element doesn't appear to be enough.
                 */
                 Ext.each([].concat(this.textNodes, this.el.query('th span')), function(el){
                     Ext.fly(el).repaint();
                 });
            }
        },

        // private
        doDisabled : function(disabled){
            this.keyNav.setDisabled(disabled);
            this.prevRepeater.setDisabled(disabled);
            this.nextRepeater.setDisabled(disabled);
            if(this.showToday){
                this.todayKeyListener.setDisabled(disabled);
                this.todayBtn.setDisabled(disabled);
            }
        },

        // private
        onRender : function(container, position){
            var m = [
                 '',
                    '  ',
                    ''],
                    dn = this.dayNames,
                    i;
            for(i = 0; i < 7; i++){
                var d = this.startDay+i;
                if(d > 6){
                    d = d-7;
                }
                m.push('', dn[d].substr(0,1), '');
            }
            m[m.length] = '';
            for(i = 0; i < 42; i++) {
                if(i % 7 === 0 && i !== 0){
                    m[m.length] = '';
                }
                m[m.length] = '';
            }
            m.push('',
                    this.showToday ? '' : '',
                    '');

            var el = document.createElement('div');
            el.className = 'x-date-picker';
            el.innerHTML = m.join('');

            container.dom.insertBefore(el, position);

            this.el = Ext.get(el);
            this.eventEl = Ext.get(el.firstChild);

            this.prevRepeater = new Ext.util.ClickRepeater(this.el.child('td.x-date-left a'), {
                handler: this.showPrevMonth,
                scope: this,
                preventDefault:true,
                stopDefault:true
            });

            this.nextRepeater = new Ext.util.ClickRepeater(this.el.child('td.x-date-right a'), {
                handler: this.showNextMonth,
                scope: this,
                preventDefault:true,
                stopDefault:true
            });

            this.monthPicker = this.el.down('div.x-date-mp');
            this.monthPicker.enableDisplayMode('block');

            this.keyNav = new Ext.KeyNav(this.eventEl, {
                'left' : function(e){
                    if(e.ctrlKey){
                        this.showPrevMonth();
                    }else{
                        this.update(this.activeDate.add('d', -1));
                    }
                },

                'right' : function(e){
                    if(e.ctrlKey){
                        this.showNextMonth();
                    }else{
                        this.update(this.activeDate.add('d', 1));
                    }
                },

                'up' : function(e){
                    if(e.ctrlKey){
                        this.showNextYear();
                    }else{
                        this.update(this.activeDate.add('d', -7));
                    }
                },

                'down' : function(e){
                    if(e.ctrlKey){
                        this.showPrevYear();
                    }else{
                        this.update(this.activeDate.add('d', 7));
                    }
                },

                'pageUp' : function(e){
                    this.showNextMonth();
                },

                'pageDown' : function(e){
                    this.showPrevMonth();
                },

                'enter' : function(e){
                    e.stopPropagation();
                    return true;
                },

                scope : this
            });

            this.el.unselectable();

            this.cells = this.el.select('table.x-date-inner tbody td');
            this.textNodes = this.el.query('table.x-date-inner tbody span');

            this.mbtn = new Ext.Button({
                text: ' ',
                tooltip: this.monthYearText,
                renderTo: this.el.child('td.x-date-middle', true)
            });
            this.mbtn.el.child('em').addClass('x-btn-arrow');

            if(this.showToday){
                this.todayKeyListener = this.eventEl.addKeyListener(Ext.EventObject.SPACE, this.selectToday,  this);
                var today = (new Date()).dateFormat(this.format);
                this.todayBtn = new Ext.Button({
                    renderTo: this.el.child('td.x-date-bottom', true),
                    text: String.format(this.todayText, today),
                    tooltip: String.format(this.todayTip, today),
                    handler: this.selectToday,
                    scope: this
                });
            }
            this.mon(this.eventEl, 'mousewheel', this.handleMouseWheel, this);
            this.mon(this.eventEl, 'click', this.handleDateClick,  this, {delegate: 'a.x-date-date'});
            this.mon(this.mbtn, 'click', this.showMonthPicker, this);
            this.onEnable(true);
        },

        // private
        createMonthPicker : function(){
            if(!this.monthPicker.dom.firstChild){
                var buf = [''];
                for(var i = 0; i < 6; i++){
                    buf.push(
                        '', Date.getShortMonthName(i), '',
                        '', Date.getShortMonthName(i + 6), '',
                        i === 0 ?
                        '' :
                        ''
                    );
                }
                buf.push(
                    '',
                    ''
                );
                this.monthPicker.update(buf.join(''));

                this.mon(this.monthPicker, 'click', this.onMonthClick, this);
                this.mon(this.monthPicker, 'dblclick', this.onMonthDblClick, this);

                this.mpMonths = this.monthPicker.select('td.x-date-mp-month');
                this.mpYears = this.monthPicker.select('td.x-date-mp-year');

                this.mpMonths.each(function(m, a, i){
                    i += 1;
                    if((i%2) === 0){
                        m.dom.xmonth = 5 + Math.round(i * 0.5);
                    }else{
                        m.dom.xmonth = Math.round((i-1) * 0.5);
                    }
                });
            }
        },

        // private
        showMonthPicker : function(){
            if(!this.disabled){
                this.createMonthPicker();
                var size = this.el.getSize();
                this.monthPicker.setSize(size);
                this.monthPicker.child('table').setSize(size);

                this.mpSelMonth = (this.activeDate || this.value).getMonth();
                this.updateMPMonth(this.mpSelMonth);
                this.mpSelYear = (this.activeDate || this.value).getFullYear();
                this.updateMPYear(this.mpSelYear);

                this.monthPicker.slideIn('t', {duration:0.2});
            }
        },

        // private
        updateMPYear : function(y){
            this.mpyear = y;
            var ys = this.mpYears.elements;
            for(var i = 1; i <= 10; i++){
                var td = ys[i-1], y2;
                if((i%2) === 0){
                    y2 = y + Math.round(i * 0.5);
                    td.firstChild.innerHTML = y2;
                    td.xyear = y2;
                }else{
                    y2 = y - (5-Math.round(i * 0.5));
                    td.firstChild.innerHTML = y2;
                    td.xyear = y2;
                }
                this.mpYears.item(i-1)[y2 == this.mpSelYear ? 'addClass' : 'removeClass']('x-date-mp-sel');
            }
        },

        // private
        updateMPMonth : function(sm){
            this.mpMonths.each(function(m, a, i){
                m[m.dom.xmonth == sm ? 'addClass' : 'removeClass']('x-date-mp-sel');
            });
        },

        // private
        selectMPMonth : function(m){

        },

        // private
        onMonthClick : function(e, t){
            e.stopEvent();
            var el = new Ext.Element(t), pn;
            if(el.is('button.x-date-mp-cancel')){
                this.hideMonthPicker();
            }
            else if(el.is('button.x-date-mp-ok')){
                var d = new Date(this.mpSelYear, this.mpSelMonth, (this.activeDate || this.value).getDate());
                if(d.getMonth() != this.mpSelMonth){
                    // 'fix' the JS rolling date conversion if needed
                    d = new Date(this.mpSelYear, this.mpSelMonth, 1).getLastDateOfMonth();
                }
                this.update(d);
                this.hideMonthPicker();
            }
            else if((pn = el.up('td.x-date-mp-month', 2))){
                this.mpMonths.removeClass('x-date-mp-sel');
                pn.addClass('x-date-mp-sel');
                this.mpSelMonth = pn.dom.xmonth;
            }
            else if((pn = el.up('td.x-date-mp-year', 2))){
                this.mpYears.removeClass('x-date-mp-sel');
                pn.addClass('x-date-mp-sel');
                this.mpSelYear = pn.dom.xyear;
            }
            else if(el.is('a.x-date-mp-prev')){
                this.updateMPYear(this.mpyear-10);
            }
            else if(el.is('a.x-date-mp-next')){
                this.updateMPYear(this.mpyear+10);
            }
        },

        // private
        onMonthDblClick : function(e, t){
            e.stopEvent();
            var el = new Ext.Element(t), pn;
            if((pn = el.up('td.x-date-mp-month', 2))){
                this.update(new Date(this.mpSelYear, pn.dom.xmonth, (this.activeDate || this.value).getDate()));
                this.hideMonthPicker();
            }
            else if((pn = el.up('td.x-date-mp-year', 2))){
                this.update(new Date(pn.dom.xyear, this.mpSelMonth, (this.activeDate || this.value).getDate()));
                this.hideMonthPicker();
            }
        },

        // private
        hideMonthPicker : function(disableAnim){
            if(this.monthPicker){
                if(disableAnim === true){
                    this.monthPicker.hide();
                }else{
                    this.monthPicker.slideOut('t', {duration:0.2});
                }
            }
        },

        // private
        showPrevMonth : function(e){
            this.update(this.activeDate.add('mo', -1));
        },

        // private
        showNextMonth : function(e){
            this.update(this.activeDate.add('mo', 1));
        },

        // private
        showPrevYear : function(){
            this.update(this.activeDate.add('y', -1));
        },

        // private
        showNextYear : function(){
            this.update(this.activeDate.add('y', 1));
        },

        // private
        handleMouseWheel : function(e){
            e.stopEvent();
            if(!this.disabled){
                var delta = e.getWheelDelta();
                if(delta > 0){
                    this.showPrevMonth();
                } else if(delta < 0){
                    this.showNextMonth();
                }
            }
        },

        // private
        handleDateClick : function(e, t){
            e.stopEvent();
            if(!this.disabled && t.dateValue && !Ext.fly(t.parentNode).hasClass('x-date-disabled')){
                this.setValue(new Date(t.dateValue));
                this.fireEvent('select', this, this.value);
            }
        },

        // private
        selectToday : function(){
            if(this.todayBtn && !this.todayBtn.disabled){
                this.setValue(new Date().clearTime());
                this.fireEvent('select', this, this.value);
            }
        },

        // private
        update : function(date, forceRefresh){
            var vd = this.activeDate, vis = this.isVisible();
            this.activeDate = date;
            if(!forceRefresh && vd && this.el){
                var t = date.getTime();
                if(vd.getMonth() == date.getMonth() && vd.getFullYear() == date.getFullYear()){
                    this.cells.removeClass('x-date-selected');
                    this.cells.each(function(c){
                       if(c.dom.firstChild.dateValue == t){
                           c.addClass('x-date-selected');
                           if(vis){
                               Ext.fly(c.dom.firstChild).focus(50);
                           }
                           return false;
                       }
                    });
                    return;
                }
            }
            var days = date.getDaysInMonth();
            var firstOfMonth = date.getFirstDateOfMonth();
            var startingPos = firstOfMonth.getDay()-this.startDay;

            if(startingPos <= this.startDay){
                startingPos += 7;
            }

            var pm = date.add('mo', -1);
            var prevStart = pm.getDaysInMonth()-startingPos;

            var cells = this.cells.elements;
            var textEls = this.textNodes;
            days += startingPos;

            // convert everything to numbers so it's fast
            var day = 86400000;
            var d = (new Date(pm.getFullYear(), pm.getMonth(), prevStart)).clearTime();
            var today = new Date().clearTime().getTime();
            var sel = date.clearTime().getTime();
            var min = this.minDate ? this.minDate.clearTime() : Number.NEGATIVE_INFINITY;
            var max = this.maxDate ? this.maxDate.clearTime() : Number.POSITIVE_INFINITY;
            var ddMatch = this.disabledDatesRE;
            var ddText = this.disabledDatesText;
            var ddays = this.disabledDays ? this.disabledDays.join('') : false;
            var ddaysText = this.disabledDaysText;
            var format = this.format;

            if(this.showToday){
                var td = new Date().clearTime();
                var disable = (td < min || td > max ||
                    (ddMatch && format && ddMatch.test(td.dateFormat(format))) ||
                    (ddays && ddays.indexOf(td.getDay()) != -1));

                if(!this.disabled){
                    this.todayBtn.setDisabled(disable);
                    this.todayKeyListener[disable ? 'disable' : 'enable']();
                }
            }

            var setCellClass = function(cal, cell){
                cell.title = '';
                var t = d.getTime();
                cell.firstChild.dateValue = t;
                if(t == today){
                    cell.className += ' x-date-today';
                    cell.title = cal.todayText;
                }
                if(t == sel){
                    cell.className += ' x-date-selected';
                    if(vis){
                        Ext.fly(cell.firstChild).focus(50);
                    }
                }
                // disabling
                if(t < min) {
                    cell.className = ' x-date-disabled';
                    cell.title = cal.minText;
                    return;
                }
                if(t > max) {
                    cell.className = ' x-date-disabled';
                    cell.title = cal.maxText;
                    return;
                }
                if(ddays){
                    if(ddays.indexOf(d.getDay()) != -1){
                        cell.title = ddaysText;
                        cell.className = ' x-date-disabled';
                    }
                }
                if(ddMatch && format){
                    var fvalue = d.dateFormat(format);
                    if(ddMatch.test(fvalue)){
                        cell.title = ddText.replace('%0', fvalue);
                        cell.className = ' x-date-disabled';
                    }
                }
            };

            var i = 0;
            for(; i < startingPos; i++) {
                textEls[i].innerHTML = (++prevStart);
                d.setDate(d.getDate()+1);
                cells[i].className = 'x-date-prevday';
                setCellClass(this, cells[i]);
            }
            for(; i < days; i++){
                var intDay = i - startingPos + 1;
                textEls[i].innerHTML = (intDay);
                d.setDate(d.getDate()+1);
                cells[i].className = 'x-date-active';
                setCellClass(this, cells[i]);
            }
            var extraDays = 0;
            for(; i < 42; i++) {
                 textEls[i].innerHTML = (++extraDays);
                 d.setDate(d.getDate()+1);
                 cells[i].className = 'x-date-nextday';
                 setCellClass(this, cells[i]);
            }

            this.mbtn.setText(this.monthNames[date.getMonth()] + ' ' + date.getFullYear());

            if(!this.internalRender){
                var main = this.el.dom.firstChild;
                var w = main.offsetWidth;
                this.el.setWidth(w + this.el.getBorderWidth('lr'));
                Ext.fly(main).setWidth(w);
                this.internalRender = true;
                // opera does not respect the auto grow header center column
                // then, after it gets a width opera refuses to recalculate
                // without a second pass
                if(Ext.isOpera && !this.secondPass){
                    main.rows[0].cells[1].style.width = (w - (main.rows[0].cells[0].offsetWidth+main.rows[0].cells[2].offsetWidth)) + 'px';
                    this.secondPass = true;
                    this.update.defer(10, this, [date]);
                }
            }
        },

        // private
        beforeDestroy : function() {
            if(this.rendered){
                this.keyNav.disable();
                this.keyNav = null;
                Ext.destroy(
                    this.leftClickRpt,
                    this.rightClickRpt,
                    this.monthPicker,
                    this.eventEl,
                    this.mbtn,
                    this.todayBtn
                );
            }
        }

        /**
         * @cfg {String} autoEl @hide
         */
    });

    Ext.reg('datepicker', Ext.DatePicker);