I'm going to cancel a date with only a couple hours notice
What are you going to tell them?
I'm so tired today. I don't want to hang out with you, hence I will cancel it.
The person has planned a date for you and you're going to back out. That is rude and inconsiderate.
They should know that I can cancel things last minute.
Don't you want to make a good impression with present and future dates? The word might go out and people might judge you negatively with it. Why not just go through with the date, they might turn out to be the one!