function my_acf_validate_save_post() {

	if ( empty( $_POST ) ) {
		return;
	}

	if ( empty( $_POST['post_ID'] ) ) {
		return;
	}

	if ( empty( $_POST['acf']['field_5dc15b87cac79'] ) || empty( $_POST['acf']['field_5dc15ca6a59c3'] ) ) {
		return;
	}

	$fxl_new_start_date = sanitize_text_field( wp_unslash( $_POST['acf']['field_5dc15b87cac79'] ) );

	$fxl_new_end_date = sanitize_text_field( wp_unslash( $_POST['acf']['field_5dc15ca6a59c3'] ) );

	// Check if user is an administrator.
	if ( current_user_can( 'manage_options' ) ) {
		// Clear all errors.
		acf_reset_validation_errors();
	}
	$fxl_pricing_page = get_page_by_title( 'Plans & Pricing' );

	if ( $fxl_pricing_page ) {

		if ( (int) $_POST['post_ID'] === $fxl_pricing_page->ID ) {// I have to use $_POST['post_ID'] here, instead of get_the_id(), because when the hook fires, $post is not defined, so get_the_id() returns false, instead of the post ID.

			if ( $fxl_new_start_date > $fxl_new_end_date ) {
				acf_add_validation_error( 'acf[field_5dc15ca6a59c3]', 'Sale end date must occur after sale start date.' );
			}
		}
	}
}
add_action( 'acf/validate_save_post', 'my_acf_validate_save_post', 10, 0 );