Can you use a cell string value in RSIBLINGS rule calc to make it dynamic off user input?

Summary:

I am working on a custom cube that allows us to track type of employee additions/reductions/promotions, transfers easily for our planning cycle. I am trying to figure out logic that lets users pick an employee date and then make sure the actual monthly amount on calculates for that period selected and future months after. To try and simplify the process I am looking to have my own time dimension and then ultimately map that back into the standard planning cube for salaries and bonus. I am having a hard time on figuring out the logic to make sure only future