{Feature Request} Increase the time span

Hi,
This is a feature request.
Under number of posts to show there is an option to show posts from the last 1,3,12 months or unlimited.

Can this number be changed or an manually entered option be made available?

For example, a client is using this plugin and has set 12 months as the setting. He set this option in 2014 so that all posts made in 2013 would start showing. But now in 2015, the 2013 posts are not showing up. He does not want to change to unlimited since posts before 2013 do not have images, so it would be great if the settings can be changed to 24 months or have an user entered option.

Thanks.

https://wordpress.org/plugins/related-posts-by-zemanta/