San Francisco is in the Pacific Time Zone (UTC−08:00), while Hong Kong is in the Hong Kong Time Zone (UTC+08:00). When flying from San Francisco to Hong Kong, you will be traveling eastward across time zones. This means that you will gain 20 hours or a full day when you arrive in Hong Kong.
For example, if you depart San Francisco at 9:00 PM on Monday and fly for 15 hours, you will arrive in Hong Kong at 10:00 AM on Wednesday. Even though it only took 15 hours to fly, you will gain a full day because of the time zone change.
