A workaround is to always select the easy "Keep [Province]" mission, and make sure you have its messages set to "popup and pause". These last 5 years, so every other mission, it's time to slide DPs again.
A couple notes:
* Missions don't show up until Feb. 1 in a new game, so you lose one month (just once)
* They tend to slide back over time (maybe a day or two each time). Not sure if this is because of one's game speed, or because a mission isn't 'complete' until the day after its end date.
Neither of the above is a biggie. Especially compared to forgetting about your DPs for years
