get_lastpostdate()
Function: Retrieves the most recent time that a post on the site was published.
Source: wp-includes/post.php:6846
get_lastpostmodified()
Function: Gets the most recent time that a post on the site was modified.
Source: wp-includes/post.php:6880
_get_last_post_time()
Function: Gets the timestamp of the last time any post was modified or published.
Source: wp-includes/post.php:6936