Yes, found it !
It isn’t very pretty, but it works (I think).
The admin page is here…
There is quite a lot going on. The top part is showing how you can display “free” dates. It is complicated by the fact that you can’t say “show me all the things that DON’T overlap” so I have a Repeating Group of things that DO overlap and then the “Free” Repeating Group filters on that list. So you are creating a list of booked rooms, and then having a list of rooms that are NOT in the that list.
The bottom part shows how you can create something like the date picker you are after. It lists out dates (you need to have a pre populated data type with dates I am afraid) and colours them if there is a booking on that date.
Hopefully the data structure makes sense, your have ROOM and that has a list of BOOKINGS. BOOKING has a Date Range. And you use that date range and OVERLAPS (for from/to) and CONTAINS (for single date) to check the dates.
Have a look and see if it makes sense.