20 CFR § 404.614 - When an application or other form is considered filed.
(a) General rule. Except as otherwise provided in paragraph (b) of this section and in §§ 404.630 through 404.633 which relate to the filing date of an application, an application for benefits, or a written statement, request, or notice is filed on the day it is received by an SSA employee at one of our offices or by an SSA employee who is authorized to receive it at a place other than one of our offices.
(b) Other places and dates of filing. We will also accept as the date of filing -
(1) The date an application for benefits, or a written statement, request or notice is received by any office of the U.S. Foreign Service or by the Veterans Administration Regional Office in the Philippines;
(2) The date an application for benefits or a written statement, request or notice is mailed to us by the U.S. mail, if using the date we receive it would result in the loss or lessening of rights. The date shown by a U.S. postmark will be used as the date of mailing. If the postmark is unreadable, or there is no postmark, we will consider other evidence of when you mailed it to us; or
(3) The date an application for benefits is filed with the Railroad Retirement Board or the Veterans Administration. See § 404.611 (b) and (c) for an explanation of when an application for benefits filed with the Railroad Retirement Board or the Veterans Administration is considered an application for social security benefits.