wp-includes/class-wp-feed-cache-transient.php:111Gets mod transient.
int public function mtime() { return get_site_transient( $this->mod_name ); }Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-feed-cache-transient.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.