Similar to "the game"[0] (you all just lost), for many years in a group of friends we'd text each other "leet" at 13:37 whenever we'd notice and whoever did it first won that day. Obvious gentleman's agreement to not just use an alarm clock and notice it naturally during the course of the day. Have been doing it for about 20 years now with one of them that kept it up.

[0] https://en.m.wikipedia.org/wiki/The_Game_(mind_game)

I'm shocked that you do this to be honest, because I have my friends and I also do this. I didn't realize that we weren't alone. Not that I thought we were special, moreso that we didn't think anyone else would want to do something so silly.

The other explanation is that you two are friends who don't know each other's usernames

Looks from the profile the other user is from Canada and I was referring to a friend group from childhood in Portugal, none of which are in Canada nowadays. I think we're just not so special :)

Used to be common on IRC as well.