My guess would be that since one traveling across an ocean like the Pacific which takes some 8 hrs from say Seattle, USA to Narita, Japan and jumping into a day forward because of the IDL. Kind of shoots the day away that you could otherwise be doing something like working, or shopping or whatever you do besides sitting on a plane for 8 hours, plus give or take another 3-4 hours getting to the airport, going through security, or getting to and from the airport so that now makes it a 12 hour day. Being you crossed an ocean, and the time difference between West Coast and Japan is a day FORWARD since you crossed the international date line, and the times are hours apart. Like day and night different. So it makes sense that a TRANSOCEANIC FLIGHT DAY.....(I've never heard it called that way) may mean dealing with time zones and International Date Line being crossed. Best way to follow all that confusion is utilizing GREENWICH MEAN TIME or (GMT) also known as COORDINATED UNIVERSAL TIME (UTC) and also known as ZULU TIME. Utilizing either one of these methods are exactly the same. World is divided into 24 time zones. Greenwich England is at 0 GMT. In the East Coast, you subtract 5 hours, (Because you are going through 5 time zones at one hour per time zone each) minus 6 hours in Central Time, 7 hours minus at Rocky Mountain Time, and so forth..... This also depends on daylights savings time which is seasonal so at times East coast is -5 hours and sometimes changes to -6 hours. and so forth.