Add 90 Days to Contract Start Date

Has anyone had success using Gainsight to add or subtract days from a date field?

I'm trying to figure out the number of users for a customer in the first 90 days of their contract. (I need to run a comparison against the number of users in the trailing 90 days from today.)

I can grab the contract start date from Opportunity, and I can join it to the Google Analytics MDA data in a Bionic Rule. Then I could limit the Google Analytics data to "Google Analytics Date >= Contract Start AND Google Analytics Date < 90 Days After Contract Start"/

However, I can't figure out a way to add 90 days to the contract start date.

• Bionic Rule > Transformation Task > +Formula Field: Formula fields don't act on Date fields, so I can't just simply add 90 Days to the Date
• Rule "Filters": I can say "This field is less/greater than another field", and I can "Add/Subtract N Days from Rule Date", but I can't "Add/Subtract N Days from Date Field"
• MDA Google Analytics Data: I could add a formula field for "This Pageview's Date plus 90 days", except that MDA object isn't editable

The only thing I can think of is adding a Salesforce Formula Field to the Opportunity, to add 90 Days to the Contract Start Date. However our Salesforce instance is busy with extra fields, so I'd rather not add yet another one unless absolutely necessary.

If you've had any luck manipulating Dates in some other way, I'd love to hear! Thanks for lending your brainpower 🙂