wp-includes/class-wp-query.php:4436Determines whether the query is for an existing month archive.
bool public function is_month() { return (bool) $this->is_month; }Introduced in 3.1.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-query.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.