Originally Posted by
corporate-wage-slave
For the USA the "clock" is not, and has never been, 24 hours. It's the day of departure or the day before departure, see the CDC website for very clear explanations of this. Personally I would make it the "day of travel or the day before travel" relating to LIS rather than LHR, if there is an overnight stay, and it's easy / cheap to do the test in LIS. But if it's one ticket and the connection is not unduly long in LIS, the rules relate to departure time in LHR rather than LIS.
In the event the query related to Portugal's requirement for transit pax to have an antigen test 24 hours "before boarding", would you know if this is 24 hours before boarding the flight to Portugal, or 24 hours before the first flight flown. I can't find clarity on this.