19 U.S.C. § 1434

(a) Formal entry Within 24 hours (or such other period of time as may be provided under subsection (c)(2)) after the arrival at any port or place in the United States of— (1) any vessel from a foreign port or place; (2) any foreign vessel from a domestic port; (3) any vessel of the United States having on board foreign merchandise for which entry has not been made; or (4) any vessel which has visited a hovering vessel or has delivered or received merchandise while outside the territorial sea; the master of the vessel shall, unless otherwise provided by law, make formal entry at the nearest customs facility or such other place as the Secretary may prescribe by regulation. (b) Preliminary entry The Secretary may by regulation permit the master to make preliminary entry of the vessel with the Customs Service in lieu of formal entry or before formal entry is made. In permitting preliminary entry, the Customs Service shall board a sufficient number of vessels to ensure compliance with the laws it enforces. (c) Regulations The Secretary may by regulation— (1) prescribe the manner and format in which entry under subsection (a) or subsection (b), or both, must be made, and such regulations may provide that any such entry may be made electronically pursuant to an electronic data interchange system; (2) provide that— (A) formal entry must be made within a greater or lesser time than 24 hours after arrival, but in no case more than 48 hours after arrival, and (B) formal entry may be made before arrival; and (3) authorize the Customs Service to permit entry or preliminary entry of any vessel to be made at a place other than a designated port of entry, under such conditions as may be prescribed.