Source

File: wp-includes/general-template.php

function get_post_modified_time( $format = 'U', $gmt = false, $post = null, $translate = false ) {
	$post = get_post( $post );

	if ( ! $post ) {
		return false;
	}

	$source   = ( $gmt ) ? 'gmt' : 'local';
	$datetime = get_post_datetime( $post, 'modified', $source );

	if ( false === $datetime ) {
		return false;
	}

	if ( 'U' === $format || 'G' === $format ) {
		$time = $datetime->getTimestamp();

		// Returns a sum of timestamp with timezone offset. Ideally should never be used.
		if ( ! $gmt ) {
			$time += $datetime->getOffset();
		}
	} elseif ( $translate ) {
		$time = wp_date( $format, $datetime->getTimestamp(), $gmt ? new DateTimeZone( 'UTC' ) : null );
	} else {
		if ( $gmt ) {
			$datetime = $datetime->setTimezone( new DateTimeZone( 'UTC' ) );
		}

		$time = $datetime->format( $format );
	}

	/**
	 * Filters the localized time a post was last modified.
	 *
	 * @since 2.8.0
	 *
	 * @param string $time   The formatted time.
	 * @param string $format Format to use for retrieving the time the post was modified.
	 *                       Accepts 'G', 'U', or PHP date format. Default 'U'.
	 * @param bool   $gmt    Whether to retrieve the GMT time. Default false.
	 */
	return apply_filters( 'get_post_modified_time', $time, $format, $gmt );
}