Hey Steve, I know this thread is a few months old, but it popped up in a search when I was looking for something similar so I wanted to update that in the Fall 2017 release we now have the functionality in the rules engine to calculate the difference between two dates. 



More info here: https://support.gainsight.com/Product_Documentation/Rules_Engine/Admin_Configuration/Formula_Builder...

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Hi Steve, Here is a good news about implementation of Formula fields in MDA, hope this solves your use-case. 



With this release, Gainsight now allows you to create calculated fields by using String and Date functions. You can find these functions on the Create Calculated Field window.



You can create a calculated field by using the existing fields in that object, as arguments of the function. Apart from the existing fields, you can also use other applicable formula functions as arguments while building a formula field.



For more information please follow this article.



Thanks for posting!

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