Meddling

If you can recall, in Ultimate Enemy, Dan took note of the time medallion around the gang's neck, and the first thing he said was, "Clockwork, meddling again." Normally, to me, this would mean that they've met somewhere down the line before the events of Ultimate Enemy. This could essentially mean a lot of things. For example, a simple time paradox. Dan could have been our Danny and had Clockwork visit him to prevent this future, and Clockwork could have failed only to try again. (sort of time paradox/dimension sort of thing.) Another thought would be that the Dan met Clockwork post transformation and Clockwork had momentarily stopped him from wreaking total annihilation on the human world or Ghost Zone (which is basically what this picture depicts.)

I also thought it would be humorous to have them both young in their first confrontation.

Dark/Daniel/Dan Phantom/Plasmius/Fenton/Masters, Clockwork  © Butch Hartman
Image © Pisces19