The challenge description tells you to play Crimson Dragon early. It's not providing any clues at what time of the day you are supposed to play the game.
I unlocked the challenge by starting the game at 8:30am (CET). I assume you will get this challenge unlocked by playing it any time in the morning.
The reward you get for the challenge is a new dragon. I am not sure if it's always the same dragon but I unlocked a "Blackbone Lazward"
Since many people posted their unlocking times in the comments, it seems to be unclear when exactly you have to start the game to unlock this challenge. Some people unlocked it in the afternoon, others in the evening or in the night.
As GreyIC and others pointed out in the comments this seems to be rather about just starting the game within the time frame of the challenge and not about the time of the day. So as soon as you just start the game while this challenge is active you should be good. If anyone posts something completely different I will change this.