The flight China to LAX is about 12 hours. There is about a 9 hour time difference between China and California.

So if you leave China at 8 am you would arrive LAX at around 5 am. Leaving at noon gets you in around 9 am.

Hard to find any 21 hour period without night.

Flying the other way, LAX to China, if you leave at noon, you arrive at 3 pm so it is an all daylight trip.

Now flying back the night is short. If you would normally expect 9 dark hours you may only get 5 or 6.

Going the other direction may work. If your destination is NYC, Air China has a 1 pm non-stop that arrives at JFK 2:30 pm some 13.5 hours later. That may be all daylight.