Wel, you don’t “gain” time, you just have more time on the ground. And you are still tired from flying in a cramped plane with artifical atmosphere and exposure to everyone’s respiratory viruses. Some airlines would charge you less for around-the-world flights. But there really are not that many “pairs” of cities. From California, opposite side cities are in Iraq/Iran/ U.A.E., and rural parts of the former Soviet Union, I have a friend who flew from San Francisco to New Delhi via Dubai because she got a deal on an Emirates flight. Most people would make that trek through Singapore. But to really gain time, one must fly East! over the international date line. I once left Hong Kong at 9 a.m., flew through Tokyo, then on to San Francisco, and landed at 7 a.m. the same morning!