Protagoras
At the beginning of Plato’s dialogue
But I see that you are disinclined, and as I have an engagement which will prevent my staying to hear you at greater length (for I have to be in another place), I will depart; although I should have liked to have heard you.
Thus I spoke, and was rising from my seat, when Callias seized me by the right hand, and in his left hand caught hold of this old cloak of mine. He said: We cannot let you go, Socrates, for if you leave us there will be an end of our discussions: I must therefore beg you to remain, as there is nothing in the world that I should like better than to hear you and Protagoras discourse. Do not deny the company this pleasure.
The dialogue concludes with these remarks, also from Socrates:
By all means, I said, if that is your wish; for I too ought long since to have kept the engagement of which I spoke before, and only tarried because I could not refuse the request of the noble Callias. So the conversation ended, and we went our way.
The excuse “I can’t stay because I have an appointment.” is at least as old as Plato.
Quotations are taken from the translation by Benjamin Jowett as published by Project Gutenberg.