What the Hell am I lookin’ at? When does this happen in the movie?

Dark Helmet: What the Hell am I lookin’ at? When does this happen in the movie?
Sandurz: Now. Whatever you’re looking at now, is happening now.
Dark Helmet: Well, what happened to then?
Sandurz: We just passed it.
Dark Helmet: When?
Sandurz: Just now.
Dark Helmet: Well, go back to then.
Sandurz: We can’t.
Dark Jelmet: Why not?
Sandurz: We already passed it.
Dark Helmet: When will then be now?
Sandurz: Soon.