While we already have World Boss tracking sites, there’s now a potentially even better way to know when they’re coming! The appropriately named Diablo 4 World Boss Timer will not only send out tweets when a World Boss is visibly marked as coming in the game, but also predictions for future start times!
There’s even discord integration as well:
As they mention in their Twitter description, this account is human-run so there may be the occasional error, not to mention the possibility of, well, people not being there, but it’s still a very useful way to get notified! Hopefully it will continue on and be very useful to many players to come.