Get BEST date? Get Closest Row?

Please excuse my last post. That was insane and i don't expect anybody to think about that crazy excuse of a problem. But if someone does have anything to offer for it that would be hugely appreciated it.

That said, and to boil down what I need.

I have two tables

Activity (many rows per ssn but no overlaping of dates)) ssn
status
effective date

Participation (many rows per snn but no overlaping of dates): ssn
plan
effective
termination

I need to write a function that returns the Earliest (BEST) termination date given some values in.

ssn, effective date, period start date

return the EARLIEST termination date.:

termination dates for those three inputs is based on:

• the next Activity effective
• the period Selection EndDate
• The next Participation Termination.