I have two tables: People, Programs

Fields in People Table: Name, InterestTimestamp, Program, Enroll Date, Completion Date, Rank
There are (4) programs: A, 1, 2, 3 (where A is a pre-program)

I currently have a script that will rank all people who are not currently enrolled, (based on each Program, quarter and timestamp). Because the programs are all 4 quarters in length, the completion date is calculated based on the program entry date that I select.

all of this works really well...but:
What I need is for this to be done dynamically. In Program 1 I have 20 enrolled. However, the each may be completing different quarters. Instead of me picking an enrollment date manually for those waiting, I am hoping that the enrollment date would be set automatically based on spots available from completions. 

For example, (3) people are expected to complete Program 1 in Fall 2017, which will open (3) spots in Program 1 for Winter 18. Currently, I would manually look at the rank list and assign the top 3 ranked people for Winter of 2018. However, I want to find a way that the enrollment date is automatically generated based on rank and open positions…

I hope that this makes sense..

I realize I need to reword and make this more clear...

I have filtered portals based on Quarter and Year, along with a summary field to get the portal count of each. If the limit each quarter is 20, how can I script so that a new person is added to the first available spot, in the first available quarter 

Why don’t you attach your file, or a mockup of it as explained here? Anatomy of a Good Topic

Have you tried looping through the portal rows? A script can do that very quickly.