I recently played a bit with the Overpass API, mainly to finally solve issue number 1 of the opening_hours_map. So I read the Overpass QL wiki page a few times and tried it. After a certain learning curve I now have understood all the basics of it. I have to say, if you work with OSM, knowing the overpass API and the ability to query for arbitrary constellations is a big advantage.
A couple of days after fixing the initial problem I thought about another use case. Since I have been working with opening_hours for some time now I came up with the idea of a query which shows facilities for which opening_hours could easily be added ;) So if your area looks all shiny on the opening_hours_map try running one of those queries ;)
The queries use a relatively new feature of the overpass API to search over keys with regular expressions: See issue 59.
Shameless self plug: If you do not know the evaluation tool yet, I would recommend to check it out since it makes this task so much easier. Just copy the opening hours from the website (check this blog post for how to find them quickly), add a few semicolons or other rule separators, copy the prettified value over to your editor and you are done …
So, lets add those missing opening_hours ;)