Appointment Plugin?

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Howdy,

So I am looking to add a booking function to my website, where a user can click a date and time to make an appointment. I did some googling and apperantly some folks were working on a plug-in for that back in 2010, but I don't think it was ever released.

So my question is, can I add a booking system to my site and how hard is it to implement this? Any tips are appreciated!