Let's not be forgetting Lieutenant Dan, the character from the book Forrest Gump and later the movie Forrest Gump and still later the namesake of the band (which was not called Forrest Gump. But you knew that).
This is certainly a much more mature nickname than Danny and a rather safe and unannoying (if that's a word) nickname for Daniel, but it should only be a nickname for Daniel, not the full first name of a person. It's too minimalist for that. There are plenty of guys called Dan, but I do have to say that the columnist Dan Savage has ruined this name for me.