In my main mod .java file, I put this in the postinit event, but not working, not throwing any error. How I must do this correctly?

		World world=Minecraft.getMinecraft().theWorld;
		world.setWorldTime(1200);

Don't change time clientside. You need to change it serverside to have an effect. Maybe use Worldloadevent or another event where you can acess the worldobj serverside.

So the code corrent, but I want to execute on the wrong place?

What...

You cannot ever get to the World object during preInit/Init/postInit, because they just does not exist!

And night is not 1200, but 12000.

 

So, subscribe to the WorldEvent.Load, and change the time there.

+ You'd better increasing tick till it is not day(!World#isDayTime()) because there are mods which changes length of day.

I did this, but not working. In my main mod file I put this under the init events:

 

	@SubscribeEvent

public void onWorldLoad(WorldEvent.Load event)
{		
	World world=MinecraftServer.getServer().worldServers[0];
		world.setWorldTime(18000);				
}

}

Try this....

@SubscribeEvent
public void onWorldLoad(WorldEvent.Load event) {
	event.world.setWorldTime(18000L);
}

 

Don`t get the Server! You can get the world from the event...

+ I'd say this again:

+ You'd better increasing tick till it is not day(!World#isDayTime()) because there are mods which changes length of day.