While building a wordpress theme for one of my client I came across some validation issues for contact form7 where two data must show logical error when check in date is bigger than the check out date. First, thing came into my mind is to edit the plugin itself, but this is not the proper solution because if client updates the plugin then my codes will broken again.
Another, solution was to use contact form 7's hooks. The filter 'wpcf7_validate_text' provides a way to directly send validation errors when your requirement is not fulfilled.
