Calculate a person's age based on a date field

gv_age.php

<?php

add_shortcode( 'gv_age', 'gv_calculate_age' );

/**
 * Calculate age in years based on entry data
 *
 * Usage inside a Custom Content field (Replace "2" with the ID of the date field):
 *
 * <code>
 * [gv_age entry_id="{entry_id}" field_id="2" /]
 * </code>
 *
 * The output will be formatted as the number of years by default. To change the output, define a custom `format`
 *
 * <code>
 * [gv_age entry_id="{entry_id}" field_id="2" format="d" /]
 * </code>
 *
 * This will now output the number of days since the date entered in the field.
 *
 * @see https://php.net/manual/en/dateinterval.format.php For options on how to format the date difference
 *
 * Will show error messages by default. Add `hide_errors="1"` to the shortcode to prevent errors from being output.
 *
 * @param $atts
 *
 * @return string
 */
function gv_calculate_age( $atts ) {

	$defaults = array(
		'field_id'    => '',
		'entry_id'    => '',
		'format'      => '%y',
		'hide_errors' => ''
	);

	$atts = shortcode_atts( $defaults, $atts, 'gv_age' );

	$entry = GFAPI::get_entry( $atts['entry_id'] );

	if ( ! $entry || is_wp_error( $entry ) ) {
		return empty( $atts['hide_errors'] ) ? 'Error: Entry not found' : '';
	}

	if ( empty( $entry[ $atts['field_id'] ] ) ) {
		return empty( $atts['hide_errors'] ) ? 'Error: Field value not specified.' : '';
	}

	$from = new DateTime( $entry[ $atts['field_id'] ] ); // Birth date
	$to   = new DateTime( 'now' );
	$interval = $from->diff( $to );

	return $interval->format( $atts['format'] ); // Default format is years ('%y')
}