37 CFR §1.110

When one or more joint inventors are named in an application or patent, the Office may require an applicant or patentee to identify the inventorship and ownership or obligation to assign ownership, of each claimed invention on its effective filing date (as defined in § 1.109) or on its date of invention, as applicable, when necessary for purposes of an Office proceeding. The Office may also require an applicant or patentee to identify the invention dates of the subject matter of each claim when necessary for purposes of an Office proceeding.