CreatedAt field type varies across entities

Some entities like `Reward`, have the `createdAt` state attribute of type `Date`, which is a Javascript which is a JS global object. Others like `Member` have the same field, but of type `Date`, which is a type alias for `number`. Some others like `Proposal` have `number | Date` where `Date` is JS Date object.

This should be handled in a uniform way, and update the `ItemMap` methods and tests for this accordingly.