New maps for ongoing, pending and finished trials

I looked at the map with all the trial locations, and realized that of course this map will just continuously fill up. Even if there would be a drastic decrease in the number of trials conducted, the map would still become fuller and fuller, because all locations were there once was a trial shows up. The map just doesn't make a difference between a - later abolished - trial in 1996/97 somewhere in Canada and an ongoing 15 year-trial only just starting now.
So I'm currently rewriting the maps to turn them into three different layers: ongoing trials, future trials (like those there the application is still pending) and finished trials.
However that leaves me with the problem that in some cases no duration of the release is given. Any ideas of how to solve that?