CreatedAt field type varies across entities

[namesty]

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.

[dkent600]

I second this request