According to United Airlines Travel Requirements:
Returning to The United States, a negative PCR-Anti/AG test is required, 24 hours of travel.
TLDR : Answer Summary
Travelers returning to the United States with United Airlines need to provide a negative PCR or Antigen test that was taken within one day prior to their flight. Clarifications from community members emphasize that 'one day' refers to a calendar day rather than a strict 24-hour window, allowing for testing the day before a flight, and suggest that antigen tests are a more affordable and quicker option for obtaining required paperwork.