There's a somewhat detailed discussion of the Latin in this song at latindiscussion.com/forum/latin/song-diem-ex-dei.9184/
The consensus seems to be that it isn't grammatically correct, or at the very least takes a lot of liberties with the Latin (probably so it flows better when sung), so creating a strict translation probably isn't possible. User "scrabulista"'s posts in the thread give a general gist of what's being said; a very loose interpretation of the title might be "a day from God," for example.
There's a somewhat detailed discussion of the Latin in this song at latindiscussion.com/forum/latin/song-diem-ex-dei.9184/ The consensus seems to be that it isn't grammatically correct, or at the very least takes a lot of liberties with the Latin (probably so it flows better when sung), so creating a strict translation probably isn't possible. User "scrabulista"'s posts in the thread give a general gist of what's being said; a very loose interpretation of the title might be "a day from God," for example.