Magento Calendar customizations

Copy this file:

\lib\Varien\Data\Form\Element\Date.php

to the local code pool:

app/code/local/Varien\Data\Form\Element\Date.php

Now you can customize the calendar setup.
To add an option for disabling past dates, change getElementHtml() function as below:

/**
     * Output the input field and assign calendar instance to it.
     * In order to output the date:
     * - the value must be instantiated (Zend_Date)
     * - output format must be set (compatible with Zend_Date)
     *
     * MMA-20141125-added new option: disable_past_dates => boolean
     * @return string
     */
    public function getElementHtml()
    {
        $this->addClass('input-text');

        $html = sprintf(
            '<input name="%s" id="%s" value="%s" %s style="width:110px !important;" />'
            .' <img src="%s" alt="" class="v-middle" id="%s_trig" title="%s" style="%s" />',
            $this->getName(), $this->getHtmlId(), $this->_escape($this->getValue()), $this->serialize($this->getHtmlAttributes()),
            $this->getImage(), $this->getHtmlId(), 'Select Date', ($this->getDisabled() ? 'display:none;' : '')
        );
        $outputFormat = $this->getFormat();
        if (empty($outputFormat)) {
            throw new Exception('Output format is not specified. Please, specify "format" key in constructor, or set it using setFormat().');
        }
        $displayFormat = Varien_Date::convertZendToStrFtime($outputFormat, true, (bool)$this->getTime());

        //MMA custom code
        $disablePastDates = $this->getDisablePastDates();
        $calendarJs = '
            <script type="text/javascript">
            //<![CDATA[
                Calendar.setup({
                    inputField: "%s",
                    ifFormat: "%s",
                    showsTime: %s,
                    button: "%s_trig",
                    align: "Bl",
                    singleClick : true';
        if($disablePastDates)
            $calendarJs .= ',disableFunc: function(date) {
                            var now= new Date();
                            if(date.getFullYear() < now.getFullYear()) { return true; }
                            if(date.getFullYear() == now.getFullYear()) { if(date.getMonth() < now.getMonth()) { return true; } }
                            if(date.getMonth() == now.getMonth()) { if(date.getDate() < now.getDate()) { return true; } }
                        }';
        $calendarJs .= '});//]]></script>';

        $html .= sprintf($calendarJs,
            $this->getHtmlId(), $displayFormat,
            $this->getTime() ? 'true' : 'false', $this->getHtmlId()
        );

        $html .= $this->getAfterElementHtml();

        return $html;
    }

Then, in your blocks, when adding a date field, you can specify in the addField options array:

'disable_past_dates' => true

If you want to change the align of the calendar, I’ve found that possible values are these, but written with lower second letter:
http://yuilibrary.com/yui/docs/api/classes/WidgetPositionAlign.html#property_BL

If you want to add a secondary date field input to a specific mass action:

$dateFormatIso = Mage::app()->getLocale()->getDateFormat(Mage_Core_Model_Locale::FORMAT_TYPE_SHORT);
            $this->getMassactionBlock()->addItem('repair_date',
                array(
                'label'=> $this->__('Set Date'),
                'url' => $this->getUrl('*/*/massRepairDate', array('_current'=>true)),
                'additional' => array(
                    'visibility_set_date' => array(  //'visibility_set_date' will be the ID of the date input field
                        'name' => 'visibility_set_date',
                        'type' => 'date',
                        'class' => 'required-entry',
                        'label' => $this->__('Select Date'),
                        'format'    => $dateFormatIso,
                        'input_format' => $dateFormatIso,
                        'gmtoffset' => true,
                        'image'    => '/skin/adminhtml/default/default/images/grid-cal.gif',
                        'disable_past_dates' => true  //this will work if you do the above customizations
                        )
                    )
                )
            );