Timeskip not yet revealed. Hoping for a flashback chapter of everyone or something (≧∀≦)
I think he left to do family/mafia work right after they fought She Li for the last time, those were still their high school days. Since He Tian promised his brother he will do and go wherever he wanted so he could get enough power to go against She Li without much losses. Now we get to see their reunion starting from He Tian staying at Mo's and finding his box of memories that were all sealed shut and had He Tians name and face blanked out.
In one of the special episodes, or right after the timeskip, when the present started to come back, Mo is pretty drunk on new years, Tian shows up and Mo tells him that he can't be real because Tian left years ago
But mentions of it in other chapters have been there, we just haven't seen it yet (but we know it happens from what they tell us lol)
When did He Tian leave bro I keep seeing ppl say he left Mo for years but like I don't even remember reading something like that