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.
Please let us know if this works for you or not.