Calculating Member Count based on Effective/Termination Date

One of my data sources is the member roster report which has effective date termination date and enrollment status.  Up until now when we had the dashboard display static (just for the current month) I used Enrollment status of ACTIVE do do a count of members for each plan.  Now that we are continuing into February and they wnt to have the dashboard more dynamic (using the slider to see different month) then the membership count would need to be based on the enrollment date and termination date (most are null).  Just wondering if anyone has any ideas on how to do this.  Thanks so much!