So, the exact phrasing is as follows.
"Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell"
Question: Would this still, to borrow the above phrase, "snap your reality suspenders" if instead it had been phrased like the following, underlined text new?
"Casting Time: 1 reaction, which you take when you would be hit by an attack or have just been targeted by the magic missile spell"
Because if it's literally just an issue with the tense of the verb, I think you may be over-thinking things.