Calculate Number of Days Between Two Dates

<?php
/**
 * Calculate Number of Days via Two Date Fields
 * http://gravitywiz.com
 */
class GWDayCount {
 
    private static $script_output;
 
    function __construct( $args ) {
 
        extract( wp_parse_args( $args, array(
            'form_id'          => false,
            'start_field_id'   => false,
            'end_field_id'     => false,
            'count_field_id'   => false,
            'include_end_date' => true,
            ) ) );
 
        $this->form_id        = $form_id;
        $this->start_field_id = $start_field_id;
        $this->end_field_id   = $end_field_id;
        $this->count_field_id = $count_field_id;
        $this->count_adjust   = $include_end_date ? 1 : 0;
 
        add_filter( "gform_pre_render_{$form_id}", array( &$this, 'load_form_script') );
        add_action( "gform_pre_submission_{$form_id}", array( &$this, 'override_submitted_value') );
 
    }
 
    function load_form_script( $form ) {
 
        $script = "new gwdc({$form['id']}, {$this->start_field_id}, {$this->end_field_id}, {$this->count_field_id}, {$this->count_adjust});";
        GFFormDisplay::add_init_script( $form['id'], 'gw_display_count', GFFormDisplay::ON_PAGE_RENDER, $script );
 
        if( self::$script_output )
            return $form;
 
        ?>
 
        <script type="text/javascript">
 
        (function($){
 
            window.gwdc = function( formId, startFieldId, endFieldId, countFieldId, countAdjust ) {
 
                this.startDateInput = $( '#input_' + formId + '_' + startFieldId );
                this.endDateInput = $( '#input_' + formId + '_' + endFieldId );
                this.countInput = $( '#input_' + formId + '_' + countFieldId );
                this.countAdjust = countAdjust;
 
                this.init = function() {
                    var gwdc = this;
                    this.populateDayCount();
                    $(document).on( 'change', '#' + this.startDateInput.attr('id') + ', #' + this.endDateInput.attr('id'), function(){
                        gwdc.populateDayCount();
                    });
                }
 
                this.getDayCount = function() {
 
                    var startDate = new Date( this.startDateInput.val() );
                    var endDate = new Date( this.endDateInput.val() );
                    var dayCount = 0;
 
                    if( !this.isValidDate( startDate ) || !this.isValidDate( endDate ) )
                        return '';
 
                    if( startDate > endDate ) {
                        return 0;
                    } else {
 
                        var diff = endDate - startDate;
                        dayCount = diff / ( 60 * 60 * 24 * 1000 ); // secs * mins * hours * milliseconds
                        dayCount = Math.round( dayCount ) + this.countAdjust;
 
                        return dayCount;
                    }
 
                }
 
                this.populateDayCount = function() {
                    this.countInput.val( this.getDayCount() ).change();
                }
 
                this.isValidDate = function( date ) {
                    return !isNaN( Date.parse( date ) );
                }
 
                this.init();
 
            }
 
        })(jQuery);
 
        </script>
 
        <?php
        self::$script_output = true;
        return $form;
    }
 
    function override_submitted_value( $form ) {
 
        $start_date = strtotime( rgpost("input_{$this->start_field_id}") );
        $end_date = strtotime( rgpost("input_{$this->end_field_id}") );
 
        if( $start_date > $end_date ) {
 
            $day_count = 0;
 
        } else {
 
            $diff = $end_date - $start_date;
            $day_count = $diff / ( 60 * 60 * 24 ); // secs * mins * hours
            $day_count = round( $day_count ) + $this->count_adjust;
 
        }
 
        $_POST["input_{$this->count_field_id}"] = $day_count;
 
    }
 
}
 
new GWDayCount( array(
	'form_id'        => 283,
	'start_field_id' => 1,
	'end_field_id'   => 2,
	'count_field_id' => 3
	) );

new GWDayCount( array(
	'form_id'        => 9,
	'start_field_id' => 1,
	'end_field_id'   => 2,
	'count_field_id' => 3
	) );

new GWDayCount( array(
	'form_id'          => 9,
	'start_field_id'   => 1,
	'end_field_id'     => 2,
	'count_field_id'   => 3,
	'include_end_date' => false
	) );