From the Introduction: "Sunset comes to a handful of communications-related sections of the USA PATRIOT [Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism] Act and the Intelligence Reform and Terrorism Prevention Act on March 10, 2006. The authority of the expiring sections remains in effect only as it relates to foreign intelligence investigations begun before sunset or to offenses or potential offenses begun or occurring before that date. Thereafter, the law reverts to its previous form unless it has been amended in the interim or subsequently renewed. The 9/11 Commission mentioned the approaching sunset and thought as a general matter that 'a full and informed debate on the PATRIOT Act would be healthy.'"
CRS Report for Congress, RS21704
Congressional Research Service: https://crsreports.congress.gov/