How do you define a charity here? Is this a simple post where users can come and like/unlike that post? Having a start and end date as a configuration setting will be global to the site and can’t be applied to specific posts. In such case we would like you to use “Exclude post/page IDs” option. Let’s say Charity 1 (post id 1) will be open for May 2015. Once May is over, you can put the post id 1 in the exclude field. So the like/unlike options will no more come up for that charity. In this way you can keep on adding more post ids to this option as months pass by.