yes this is usual for wix, unfortunately. simple request, complicated solution; I needed to set their countdown timer based on a future date from a database row. I had to build my own timer and message function to accommadate ( 15 minute job in a normal dev environment, 2 hour job in wix!!)
however i will say $900 is a little steep but i have not seen your set up;
you would need a custom database to hold your non dates, likely a custom element or html frame on page and a calendar plugin of some sort to go inside; most javascript involved plugins, like jquerys one, have an option to provide an array of dates not available;