Gravity Forms: Validate two date fields to ensure the "End date" is after the "Start date"

gform_validate_two_date_fields.php

<?php

// Form #215
// Start date field id: #18
// End date field id: #19
// Validation only applies to the end date, but checks the value of both dates
add_filter('gform_field_validation_215_19', 'rs_validate_two_gf_date_fields', 10, 4);

function rs_validate_two_gf_date_fields( $result, $value, $form, $field ) {
	$start_date_field_id = 18;
	$end_date_field_id = 19;

	$start_date = rgpost("input_{$start_date_field_id}");
	$end_date = rgpost("input_{$end_date_field_id}");

	$end_date_value = new DateTime(GFFormsModel::prepare_date( $field->dateFormat, array_values($end_date) ));
	$start_date_value = new DateTime(GFFormsModel::prepare_date( $field->dateFormat, array_values($start_date) ));

	if ( $end_date_value < $start_date_value ) {
		$result['is_valid'] = false;
		$result['message'] = 'End Date must be on or after the Start date.';
	}

	return $result;
}