wp-includes/class-wp-rewrite.php:576Retrieves the month permalink structure without day and with year.
string|false public function get_month_permastruct() { $structure = $this->get_date_permastruct(); if ( empty( $structure ) ) { return false; } $structure = str_replace( '%day%', '', $structure ); $structure = preg_replace( '#/+#', '/', $structure ); return $structure; }Introduced in 1.5.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/class-wp-rewrite.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.