34 C.F.R. § 300.196
(b) The filing date under paragraph (a) of this section is the date the document is—
(f) A party must show a proof of mailing to establish the filing date under paragraph (b)(2) of this section as provided in 34 CFR 75.102(d).
(Authority: 20 U.S.C. 1412(f)(3))