Time Decay Formula for Conversion Attribution
Here's one solution:

using the following data:

USER-LEVEL:


fss first session start (timestamp) - time of first pageview of
first session for that user
css converting session start (timestamp) - time of the first pageview of session where user converted
ns number of sessions (int) - total # of sessions
before conversion for user


SESSION-LEVEL:


ss session start (timestamp) - timestamp of first pageview in session


FORMULA: calculated for each session


FOR FIRST SESSION: Attribution Credit is just linear: (or really whatever you want to make it)  
ALL OTHER SESSIONS: 


First calculate the SUM of all session's "time before conversion":
Then Calculate the credit for each session:



it isn't perfect because it distributes credit based on the relative position of a session in the User's lifetime.  Ideally you have a preset halflife (like 7 days) so that comparison is more consistent.