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We just adopted the Mongo query syntax since it's easy to read and write. Our SDKs have a custom built rules engine that supports a subset of that query language. I have a feeling that `$expr` might be difficult to implement and cause the size of the SDKs to increase substantially.

For relative time operations, the easiest solution right now is to add a dynamic attribute like `daysSinceSignUp`

I can see us adding a preprocessing step in the future that let's you reference some dynamic variables like $$NOW. So the query could be complex and dynamic, but the end result being passed into the SDK would still be simple and not need $expr.

Got it, thaaanks! The `daysSinceSignUp` sounds like a nice solution