From Sales Order -> Invoice the [Date] field should be passed on and validated for the posting period. How can I retrieve the id:trandate from the Sales Order and pass it as an Date object to the invoice? I'm using N/search to get the value via the internal id, but it returns an object and not a date object. Is it easier to convert the object into a date object or is there another way to get the date object from the sales order?

depending on how you did the search, you should eventually be able to get a string representing the Date from the search results (or lookup object)