So when they reset it, it then gives you your "new" total CDP, correct? I keep a little notepad next to my PC and jot down little notes like this, how many ranked rounds I had when I last tiered up, etc... So next time it resets, just jot down how many days and on what date, then if/when it happens again you can use Scott's method above and come up with a number accurate to within day or 2.