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Have you considered adding a "Condition" into the mix. A "Condition" tool would allow the "Action" to be executed only if the condition is met (i.e. when user checks "Custom" and chooses a date). And then the "Default" selection doesn't need to execute any action.
For Actions that update expressions I find it easier to "Update Value" and "Replace a specific string", rather than try to re-write the entire expression (I think because of the need for all sorts of nested quotation marks):