I do not know which one is right for get UTC time
My code is
System.currentTimeMillis()
for java android
Is the result correct for international?
Maybe user can change device time and result be different? (Does it affect on UTC?)
On Linux platform, the system clock should be set to UTC. Whether it actually is, and whether it is accurate, is ultimately up to the user.
Calling System.currentTimeMillis() will give the time in UTC since 1970-01-01T00:00:00Z.
Is the result correct for international?
Yes. Provided that the clock is synced with a decent network time source and the user hasn't messed with it.
Maybe user can change device time and result be different?
Yes they can. There is not much you can do about it.
You could attempt to connect to a network time server, but the user could block that, or cause your game to connect to a fake time server. If they "own" the platform that your game runs on, you probably have no way to get guaranteed reliable time.
(The Wikipedia page on NTP talks about man-in-the-middle attacks. Unfortunately, standard NTP doesn't have a way to deal with that. There is a draft RFC for Network Time Security, but they have been working on it since 2015, and it still hadn't concluded at the time of writing.)