A relationship that matches to an unstored calculation does not work.
Thanks Phil, I've set up an auto date enter on the line items records and then a calculation from that in the same table to get the year. This is then in a relationship with a "get current year" global field in the home table.
If a user changes an invoice date after making it / adding line items records to it then I've scripted on save to go to related line items records and update the creation date.
This works, but is it ideal?
Could very well be best that you can do.