haven't read 19days yet but there are guys who say that the two are similar;
http://www.mangago.zone/read-manga/bromance/
lmao i started reading now and it's really similar, even the artstyle! Ahahahahaha
Tamen De Gushi
Oh also A lucky coin
this isn't similar to 19 days at all but still very good
http://www.mangago.zone/read-manga/nan_hao_shang_feng/ not really shounen ai but somewhat similar
Can someone link any manhwas that are similar to 19 days please