Average Daily Subscription over time period

Thank you so much for your post.   This is a step in the right direction for sure.     

Let me do a better job (hopefully) of explaining what I am trying to do.    We have a situation where people can enroll or drop classes at any point.   They may be enrolled in multiple different classes at the same time, or they may drop one class and add another mid-month.

What I am trying to come up with is a way to find the average number of enrollments that were active per day over a given time period.    Basically, if the time period is a month, adding up how many were active on the 1st, how many were active on the second, .... all the way to the end of the month - then dividing by the number of days. 

I can get the total enrollments that were active at any point (which was my measure listed above.)   If someone drops one class and enrolls in another, that counts them twice when they were only ever active in a single class at once.  I can also count the number of unique students that were enrolled, but this doesn't reflect when a student may be enrolled in multiple classes at once.

I need a way to iterate through the days in the time period, adding up the active enrollments for each day, then dividing that total by the number of days.   I can get the number of days (as the above post has helpfully shown), I just can't seem to figure out how to get the sum of each separate day.