"UA869 SFO-HKG: The maintenance delay on your aircraft caused members of your crew to exceed the maximum number of working hours allowed by the FAA, and we need to assign your flight new crew members. At this time, we are now estimating a 10:00pm departure to allow for your new crew members to arrive. We sincerely apologize for this disruption to your travel plans, and we appreciate your continued patience."

As a result, UA878 HKG-SFO on 12/31 delayed 13.5 hours to 1/1/2025: "Flight UA878 from Hong Kong (HKG) to San Francisco is delayed further because your crew has not had sufficient rest time required by federal law. It now departs at 12:00pm on January 1."