The two answers so far have missed the point of the question. Currently there is no way to to get a calendar file of dates needed to complete a challenge except for one - caching for every day of the year. The needed found dates would not help for other challenges such as Find one cache on the 1st, 2 on the 2nd, 3 on the 3rd, etc.
t would seem to me very unlikely such a tool would be written. The best way would be to add this functionality into the challenge checker system, so the checker system would need a facility whereby it could export the needed dates, and the checker writer would have to modify their checker to take advantage of this facility. It would also require every checker to automatically run every day (so would be a paid membership facility only) to process updates, which might be a significant load on the servers.
So not only is there work at PGC, but all the checker writers would have to get on board too. The problem is that so few challenges work off dates and it is hard to see how this could be adapted to suit other types of challenges.