The way it was explained to me (by company publication) was if you went on a flight like DTW-FRA, that spans two days, you will use 2 trans oceanic days. So if you went LAX-SYD, you would use 3 days thanks to crossing the international date line. Granted the return which departs and arrives the same day only uses 1 day.

Just go by a ZED on a foreign carrier, It'll probably cost less and you will probably get better service.