This chapter shows that it was after 3rd year of High-School, but we've known he went away for sure since the previous Christmas specials. Future-Mo has always treated He Tian with this much disregard because of it; when he got drunk, he asked where He Tian had gone, and He Tian said he was overseas. If you rememnder the deal he made with his brother (and father I'd presume), this was probably because of that
In chapter 412 He Tian explains himself (going overseas), and chapter 224 is the first Christmas special his disappearance gets mentioned. When they were dealing with She Li, He Tian wanted help to take on She Li's gang, so that he could get a 1-on-1 without the underlings getting in his way-- and so he ends up asking his brother to "lend him some of his men". This ends in an exchange where he says he'll stop trying to avoid their dad (who wants to convince He Tian to take on their shady family business) so that He Cheng will help him. In 381, after he beats up She Li, there's some mention of He Tian not being allowed to back off of his (shady) family business-- which imples his father was either somewhat successful in changing He Tian's mind or that this conversation showed He Tian he couldn't run from it forever.
I don't really blame for not remembering all of that lol but I keep tabs on almost all of the plot points so far
I thought they were always together? When did he tian leave?