Well, it's hard to find the flight time but from what i can find it's around 17hours !
Now i'm typing i realise some quick Math would be easier. The distance can easily be found, 8100 miles. Boeing 737's have a cruising speed of 485mph (assuming its a similar plane they use, of course)
8100 / 485 = 16.7hours flight time. Owch. Very few planes can fly that far, especially at full load. From some further research none of the planes in federal express's fleet can accomplish this, barely 2/3rds of the distance in fact. Which is somewhat confusing, they must have to stop somewhere (but that screws up my idea of indirect/direct flights).
Anyway, Hong Kong is +8hours and i think Memphis is -5 hours. So if the flight takes off at 8pm at night... 8pm at night is 7am in the morning in Memphis, so it'd arrive 12noon there taking into account the change in time zones.
My math is not great, i am sure i went wrong somewhere. But you get the idea

Calculating travel times when you cross time zones confuses me