Rolling 12 Months Calculation

Summary

Rolling 12 Months Calculation

Content

I need to figure out how to get a rolling 12 months on my report.  I was using MSUM function, but the numbers seem close but not quite right.  I also saw a PERIODROLLING function, i tried that, but i must not be doing something right.  Which of those two should i be using, or should i use a differernt function to figure out a ROLLING 12 Months?

my MSUM calc is:     MSUM((COUNT(*)), 11)

I highlighted in my image, the total is 695 for Nov 2018 but my database says 714.  and if i do a straight count report i get 714.