Your application may entitle you to benefits for up to 6 months or 12 months (depending on the type of benefit, as explained in § 404.621 ) before the month in which it is filed. You may limit the number of months of your entitlement in the 6-month or 12-month period. You may state this choice any time before a decision is made on your claim by indicating, in writing, the month you want your benefits to begin. You may change the first month of entitlement in this 6-month or 12-month period after a decision has been made on your claim under the following conditions:
20 C.F.R. §404.622