In 1966 President Lyndon Johnson signed into law the Uniform Time Act, which stated that daylight-saving time must begin on the last Sunday of April and end on the last Sunday of October. The law does not require that everyone follow daylight-saving time. Rather, it establishes a uniform set of dates by which to abide. States may opt to exclude themselves from the time change. The federal law has since been amended to change the dates of the time change. Today daylight-saving time begins on the second Sunday of March and ends on the first Sunday of November. With the exception of Hawaii and most of Arizona, all U.S. states currently observe daylight-saving time.
What did the Uniform Time Act do?

It repealed daylight-saving time.

It made daylight-saving time mandatory.

It standardized start and end dates for daylight-saving time.

It offered incentives to states that adopted daylight-saving time.