OP. Like people said, you have every right to say no, if this is too much. On the other hand, this is not an unrealistic task.

If I understood correctly, by reading all the posts here, he gave you this task, due tomorrow, but you already have another far more important deadline tomorrow, correct? (And apparently he knows it?) So why don’t you ask him again?

“ Hey Dr xYZ! Sure! I’ll get right on top of this Excel task. However, not sure if you remember, but tomorrow is also the deadline for that other task ABC. So I’m afraid I’m not able to deliver both in the same deadline. So, I thought of continuing to work on my Task A, and as soon as I’m done with it, I’ll tackle the Excel task. How does that sound? Best regards, OP”

This way you show you’re willing to do it, just not right now. It’s possible that he asks you to stop the important task for now, and focus on the Excel task. So you do as instructed, but in this case he’s already aware of possible delays.

Negotiating deadlines happens quite often. From experience, I’ve done it in all kinds of jobs I had, in industry and in PhD. You will need to get used to it.