Calculate days between two date fields

gv_calculate_days.php

<?php
add_shortcode( 'gv_days', 'gv_calculate_days' );
function gv_calculate_days( $atts ) {
	global $gravityview_view;
	extract($gravityview_view->field_data); // create a $entry variable with current entry data array

	extract( shortcode_atts(
		array(
			'start_date_id' => '',
			'end_date_id' => '',
		), $atts )
	);

	$start_date = $entry[$start_date_id];
	$end_date = $entry[$end_date_id];
	
    if (empty($start_date) || empty($end_date)) {
        return 'Error: No field ID specified.';
    }else{
		$start = new DateTime($start_date);
	    $end = new DateTime($end_date);
	    $diff = $start->diff($end)->days;
    }

	return $diff;
}

//example usage inside a Custom Content field: [gv_days start_date_id="2" end_date_id="3"]

@offordscott try with this code: add_filter( 'gravityview/fields/custom/decode_shortcodes', '__return_true' );