The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */
    /**
     * @class Ext.form.DateField
     * @extends Ext.form.TriggerField
     * Provides a date input field with a {@link Ext.DatePicker} dropdown and automatic date validation.
     * @constructor
     * Create a new DateField
     * @param {Object} config
     * @xtype datefield
     */
    Ext.form.DateField = Ext.extend(Ext.form.TriggerField,  {
        /**
         * @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} altFormats
         * Multiple date formats separated by "|" to try when parsing a user input value and it
         * does not match the defined format (defaults to
         * 'm/d/Y|n/j/Y|n/j/y|m/j/y|n/d/y|m/j/Y|n/d/Y|m-d-y|m-d-Y|m/d|m-d|md|mdy|mdY|d|Y-m-d').
         */
        altFormats : "m/d/Y|n/j/Y|n/j/y|m/j/y|n/d/y|m/j/Y|n/d/Y|m-d-y|m-d-Y|m/d|m-d|md|mdy|mdY|d|Y-m-d",
        /**
         * @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 {String} minText
         * The error text to display when the date in the cell is before {@link #minValue} (defaults to
         * 'The date in this field must be after {minValue}').
         */
        minText : "The date in this field must be equal to or after {0}",
        /**
         * @cfg {String} maxText
         * The error text to display when the date in the cell is after {@link #maxValue} (defaults to
         * 'The date in this field must be before {maxValue}').
         */
        maxText : "The date in this field must be equal to or before {0}",
        /**
         * @cfg {String} invalidText
         * The error text to display when the date in the field is invalid (defaults to
         * '{value} is not a valid date - it must be in the format {format}').
         */
        invalidText : "{0} is not a valid date - it must be in the format {1}",
        /**
         * @cfg {String} triggerClass
         * An additional CSS class used to style the trigger button.  The trigger will always get the
         * class 'x-form-trigger' and triggerClass will be appended if specified
         * (defaults to 'x-form-date-trigger' which displays a calendar icon).
         */
        triggerClass : 'x-form-date-trigger',
        /**
         * @cfg {Boolean} showToday
         * false to hide the footer area of the DatePicker containing the Today button and disable
         * the keyboard handler for spacebar that selects the current date (defaults to true).
         */
        showToday : true,
        /**
         * @cfg {Date/String} minValue
         * The minimum allowed date. Can be either a Javascript date object or a string date in a
         * valid format (defaults to null).
         */
        /**
         * @cfg {Date/String} maxValue
         * The maximum allowed date. Can be either a Javascript date object or a string date in a
         * valid format (defaults to null).
         */
        /**
         * @cfg {Array} disabledDays
         * An array of days to disable, 0 based (defaults to null). Some examples:
    // disable Sunday and Saturday:
    disabledDays:  [0, 6]
    // disable weekdays:
    disabledDays: [1,2,3,4,5]
         *
         */
        /**
         * @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:
    // disable these exact dates:
    disabledDates: ["03/08/2003", "09/16/2003"]
    // disable these days for every year:
    disabledDates: ["03/08", "09/16"]
    // only match the beginning (useful if you are using short years):
    disabledDates: ["^03/08"]
    // disable every day in March 2006:
    disabledDates: ["03/../2006"]
    // disable every day in every March:
    disabledDates: ["^03"]
         *
         * 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 {@link #format date format} that has "." in
         * it, you will have to escape the dot when restricting dates. For example: ["03\\.08\\.03"].
         */
        /**
         * @cfg {String/Object} autoCreate
         * A {@link Ext.DomHelper DomHelper element specification object}, or true for the default element
         * specification object:
         * autoCreate: {tag: "input", type: "text", size: "10", autocomplete: "off"}
         *
         */

        // private
        defaultAutoCreate : {tag: "input", type: "text", size: "10", autocomplete: "off"},

        // in the absence of a time value, a default value of 12 noon will be used
        // (note: 12 noon was chosen because it steers well clear of all DST timezone changes)
        initTime: '12', // 24 hour format

        initTimeFormat: 'H',

        // PUBLIC -- to be documented
        safeParse : function(value, format) {
            if (/[gGhH]/.test(format.replace(/(\\.)/g, ''))) {
                // if parse format contains hour information, no DST adjustment is necessary
                return Date.parseDate(value, format);
            } else {
                // set time to 12 noon, then clear the time
                var parsedDate = Date.parseDate(value + ' ' + this.initTime, format + ' ' + this.initTimeFormat);

                if (parsedDate) return parsedDate.clearTime();
            }
        },

        initComponent : function(){
            Ext.form.DateField.superclass.initComponent.call(this);

            this.addEvents(
                /**
                 * @event select
                 * Fires when a date is selected via the date picker.
                 * @param {Ext.form.DateField} this
                 * @param {Date} date The date that was selected
                 */
                'select'
            );

            if(Ext.isString(this.minValue)){
                this.minValue = this.parseDate(this.minValue);
            }
            if(Ext.isString(this.maxValue)){
                this.maxValue = this.parseDate(this.maxValue);
            }
            this.disabledDatesRE = null;
            this.initDisabledDays();
        },

        initEvents: function() {
            Ext.form.DateField.superclass.initEvents.call(this);
            this.keyNav = new Ext.KeyNav(this.el, {
                "down": function(e) {
                    this.onTriggerClick();
                },
                scope: this,
                forceKeyDown: true
            });
        },

        // private
        initDisabledDays : function(){
            if(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} disabledDates An array of date strings (see the {@link #disabledDates} config
         * for details on supported values) used to disable a pattern of dates.
         */
        setDisabledDates : function(dd){
            this.disabledDates = dd;
            this.initDisabledDays();
            if(this.menu){
                this.menu.picker.setDisabledDates(this.disabledDatesRE);
            }
        },

        /**
         * 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;
            if(this.menu){
                this.menu.picker.setDisabledDays(dd);
            }
        },

        /**
         * Replaces any existing {@link #minValue} with the new value and refreshes the DatePicker.
         * @param {Date} value The minimum date that can be selected
         */
        setMinValue : function(dt){
            this.minValue = (Ext.isString(dt) ? this.parseDate(dt) : dt);
            if(this.menu){
                this.menu.picker.setMinDate(this.minValue);
            }
        },

        /**
         * Replaces any existing {@link #maxValue} with the new value and refreshes the DatePicker.
         * @param {Date} value The maximum date that can be selected
         */
        setMaxValue : function(dt){
            this.maxValue = (Ext.isString(dt) ? this.parseDate(dt) : dt);
            if(this.menu){
                this.menu.picker.setMaxDate(this.maxValue);
            }
        },

        /**
         * Runs all of NumberFields validations and returns an array of any errors. Note that this first
         * runs TextField's validations, so the returned array is an amalgamation of all field errors.
         * The additional validation checks are testing that the date format is valid, that the chosen
         * date is within the min and max date constraints set, that the date chosen is not in the disabledDates
         * regex and that the day chosed is not one of the disabledDays.
         * @param {Mixed} value The value to get errors for (defaults to the current field value)
         * @return {Array} All validation errors for this field
         */
        getErrors: function(value) {
            var errors = Ext.form.DateField.superclass.getErrors.apply(this, arguments);

            value = this.formatDate(value || this.processValue(this.getRawValue()));

            if (value.length < 1) { // if it's blank and textfield didn't flag it then it's valid
                 return errors;
            }

            var svalue = value;
            value = this.parseDate(value);
            if (!value) {
                errors.push(String.format(this.invalidText, svalue, this.format));
                return errors;
            }

            var time = value.getTime();
            if (this.minValue && time < this.minValue.getTime()) {
                errors.push(String.format(this.minText, this.formatDate(this.minValue)));
            }

            if (this.maxValue && time > this.maxValue.getTime()) {
                errors.push(String.format(this.maxText, this.formatDate(this.maxValue)));
            }

            if (this.disabledDays) {
                var day = value.getDay();

                for(var i = 0; i < this.disabledDays.length; i++) {
                    if (day === this.disabledDays[i]) {
                        errors.push(this.disabledDaysText);
                        break;
                    }
                }
            }

            var fvalue = this.formatDate(value);
            if (this.disabledDatesRE && this.disabledDatesRE.test(fvalue)) {
                errors.push(String.format(this.disabledDatesText, fvalue));
            }

            return errors;
        },

        // private
        // Provides logic to override the default TriggerField.validateBlur which just returns true
        validateBlur : function(){
            return !this.menu || !this.menu.isVisible();
        },

        /**
         * Returns the current date value of the date field.
         * @return {Date} The date value
         */
        getValue : function(){
            return this.parseDate(Ext.form.DateField.superclass.getValue.call(this)) || "";
        },

        /**
         * Sets the value of the date field.  You can pass a date object or any string that can be
         * parsed into a valid date, using {@link #format} as the date format, according
         * to the same rules as {@link Date#parseDate} (the default format used is "m/d/Y").
         *
    Usage:
         *
    //All of these calls set the same date value (May 4, 2006)

    //Pass a date object:
    var dt = new Date('5/4/2006');
    dateField.setValue(dt);

    //Pass a date string (default format):
    dateField.setValue('05/04/2006');

    //Pass a date string (custom format):
    dateField.format = 'Y-m-d';
    dateField.setValue('2006-05-04');

         * @param {String/Date} date The date or valid date string
         * @return {Ext.form.Field} this
         */
        setValue : function(date){
            return Ext.form.DateField.superclass.setValue.call(this, this.formatDate(this.parseDate(date)));
        },

        // private
        parseDate : function(value) {
            if(!value || Ext.isDate(value)){
                return value;
            }

            var v = this.safeParse(value, this.format),
                af = this.altFormats,
                afa = this.altFormatsArray;

            if (!v && af) {
                afa = afa || af.split("|");

                for (var i = 0, len = afa.length; i < len && !v; i++) {
                    v = this.safeParse(value, afa[i]);
                }
            }
            return v;
        },

        // private
        onDestroy : function(){
            Ext.destroy(this.menu, this.keyNav);
            Ext.form.DateField.superclass.onDestroy.call(this);
        },

        // private
        formatDate : function(date){
            return Ext.isDate(date) ? date.dateFormat(this.format) : date;
        },

        /**
         * @method onTriggerClick
         * @hide
         */
        // private
        // Implements the default empty TriggerField.onTriggerClick function to display the DatePicker
        onTriggerClick : function(){
            if(this.disabled){
                return;
            }
            if(this.menu == null){
                this.menu = new Ext.menu.DateMenu({
                    hideOnClick: false,
                    focusOnSelect: false
                });
            }
            this.onFocus();
            Ext.apply(this.menu.picker,  {
                minDate : this.minValue,
                maxDate : this.maxValue,
                disabledDatesRE : this.disabledDatesRE,
                disabledDatesText : this.disabledDatesText,
                disabledDays : this.disabledDays,
                disabledDaysText : this.disabledDaysText,
                format : this.format,
                showToday : this.showToday,
                minText : String.format(this.minText, this.formatDate(this.minValue)),
                maxText : String.format(this.maxText, this.formatDate(this.maxValue))
            });
            this.menu.picker.setValue(this.getValue() || new Date());
            this.menu.show(this.el, "tl-bl?");
            this.menuEvents('on');
        },

        //private
        menuEvents: function(method){
            this.menu[method]('select', this.onSelect, this);
            this.menu[method]('hide', this.onMenuHide, this);
            this.menu[method]('show', this.onFocus, this);
        },

        onSelect: function(m, d){
            this.setValue(d);
            this.fireEvent('select', this, d);
            this.menu.hide();
        },

        onMenuHide: function(){
            this.focus(false, 60);
            this.menuEvents('un');
        },

        // private
        beforeBlur : function(){
            var v = this.parseDate(this.getRawValue());
            if(v){
                this.setValue(v);
            }
        }

        /**
         * @cfg {Boolean} grow @hide
         */
        /**
         * @cfg {Number} growMin @hide
         */
        /**
         * @cfg {Number} growMax @hide
         */
        /**
         * @hide
         * @method autoSize
         */
    });
    Ext.reg('datefield', Ext.form.DateField);