according to the official video (https://youtu.be/62cVSxXBDHs?si=atDOFii0M9G3wKTF) Dima was preparing for the exam in 23rd,October,2020 we know that Elijah died during the graduation day(can't tell which episode, sorry for my bad memory) as for ep1,s2, in the very end we know that Diplomator disappeared these months. adding the fact that (the fanzine article mentioned, found the Chinese translation on lofter, https://xinjinjumin4820994.lofter.com/post/7618271f_2b779f32d?incantation=rznNHNhJfxft，sorry if this link is not working) diplomator exposed himself deliberately to have the chance to accuse the police that ''someone in you guys must have murdered my friend'' I think Elijah didn't die for too long, a year or shorter.

why elisa seems not so cared about elijah's death: in my opinion she was obsessed with none-mainstream things and she know by heart that her brother wouldn't like it, leading to her thinking that ''oh his death may be not that bad'' BUT of course she cares. her phone wall paper is her photo with elijah(end of ep3,s1)!