Rule Help: Reset Count Field with new year

We're in the process of implementing some tiered service for our Success accounts.  In doing so, I've built out fields on Customer Info to store the number of allotted activities based on the level of account.  I've also built out fields to store the number of completed activities as well.  I'm populating both via the Rules Engine.  The problem I'm running into is that some of our contracts are multi-year, and I need to reset the count of completed activities each year.  So, for example, if a three contract is signed today, I want to count the number of completed activities up to this point next year, and then, I want the count to restart.  



To give some additional info:  I'm using a field we have in SF for Largest Contract Start date to indicate when to start the count, but that value won't change (in the case above from today) until the contract renews.  I was thinking of potential creating a "Days since Largest Contract Start Date" field, and then, resetting based on that field reaching 365.



Anyone have any other ideas that might not require the creation of a new field?  Or has anyone done something similar a different way?  Thanks!