Danny - are you using a CRM? You could potentially create a formula field that calculates the month value based on another value (expiry date for instance) in your CRM.
Because Marketo has trouble reading formula fields in the CRM, you would then need an additional field so that the CRM could post the calculated month value to it, so that Marketo could pick it up. You would just run this calculation as a batch every 24 hours (or more, or less). I know I used to do this in Salesforce all the time.