Similar? Hmm..that would be hard... T﹏T I could recommend u some bromantic manga tho..
http://www.mangago.zone/read-manga/orenchi_no_furo_jijou/
http://www.mangago.zone/read-manga/flat_aogiri_natsu/
http://www.mangago.zone/read-manga/otomen/
(It's a hetero romance but there is lots of bromance moments.)
This one's very similar to 19 days, as in, it's so cute and random:
http://www.mangago.zone/read-manga/hey_your_cat_ears_are_showing/
This one's shoujo-ai but is very much similar to 19 days since it's written by tan jiu, who's old Xian's friend.
http://www.mangago.zone/read-manga/tamen_di_gushi/
You should read all of tan jiu and old Xian's works, all of them are awesome.
I really hope you'll read the ones I've mentioned, since I don't want anyone missing out on these awesome works.
Maybe this might help i actually made this list for me after reading 19 day. And " oh yea that one is the best " . U can take a look http://www.mangago.zone/home/mangalist/45946/ .and also all of them r not yaoi .
I have just finished to read this manga and my feels ugh.... Just love it ! Can someone recommend me a manga similar to this ? Involving bromance something like that. And does someone know if it exists 19 days in fanfic ?