Is it possible, trying to do this natively, to tell the system after X date, all transactions need to go to subsidiary 2, while before that date the transactions need to go to subsidiary 1?

I think the customization of the form wont allow you to do it; and since the field it's standard from NetSuite you wont be able to add a formula as a default value. You can do it pretty simple with a suitescript userevent on the beforeload, or maybe a workflow.

Thanks Martin, yep, was trying to see if there was a way without SS before going all in.

instead of the date?

It gets messy. I was thinking on X date, set subsidiary to new sub for the employee, then default to own at role level.