Shortcode that receives a date field and calculates it with another date using relative dates

gv-expiration.php

<?php // DO NOT COPY THIS LINE

//USAGE: [gv_expiration date="2" duration="+1 year"]
// 2 is the field ID of the date field
// If original date is "2021-04-01" it will output "2022-04-01"
// https://docs.gravityview.co/article/79-using-relative-start-dates-and-end-dates

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

	extract( shortcode_atts(
		array(
			'date' => '',
			'duration' => '',
		), $atts )
	);

	$date = $entry[$date];
	
    if (empty($date) || empty($duration)) {
        return 'Error: No field ID specified.';
    }else{
	    $expiration = date('Y-m-d', strtotime($date.' '.$duration));
    }

	return $expiration;
}