17 C.F.R. § 240.14a-10
Prohibition of certain solicitations
No person making a solicitation which is subject to §§ 240.14a-1 to 240.14a-10 shall solicit:
(a) Any undated or postdated proxy; or
(b) Any proxy which provides that it shall be deemed to be dated as of any date subsequent to the date on which it is signed by the security holder.