Source

File: wp-includes/rest-api/endpoints/class-wp-rest-posts-controller.php

protected function prepare_date_response( $date_gmt, $date = null ) {
		// Use the date if passed.
		if ( isset( $date ) ) {
			return mysql_to_rfc3339( $date );
		}

		// Return null if $date_gmt is empty/zeros.
		if ( '0000-00-00 00:00:00' === $date_gmt ) {
			return null;
		}

		// Return the formatted datetime.
		return mysql_to_rfc3339( $date_gmt );
	}