26 USC 6071 - Time for filing returns and other documents

(a) General rule 
When not otherwise provided for by this title, the Secretary shall by regulations prescribe the time for filing any return, statement, or other document required by this title or by regulations.
(b) Electronically filed information returns 
Returns made under subparts B and C of part III of this subchapter which are filed electronically shall be filed on or before March 31 of the year following the calendar year to which such returns relate.
(c) Special taxes 
For payment of special taxes before engaging in certain trades and businesses, see section 4901 and section 5142.