The question is two parts, really. First, is it one or two sentences? I'd say two. Sentence one is the announcement of the time. Sentence two is an imperative with an understood subject, 'you.'
Second, should either or both have an exclamation point? I'm aware that it's often overused, and when it is, its impact is reduced to almost nothing. If the situation is life-or-death, literally, I'd stick with the exclamation point, but if it's mere lateness, whether it belongs is up to you, depending on how many others appear in your work.
Third (which is a neat trick for a two-part question), I think if you choose to keep the exclamation point, I screamed becomes redundant. You could go with no dialogue attribution at all, or use an action by the character instead if you need to make it clear who's speaking.
“It’s 7:20. Hurry!” I start the engine. works for me.
Maryn, almost never late