I left the party earlier and heard stories of Jay spending the whole night drawing for people. It's strange that he has two dates listed. I don't remember paying Jay for it either.
Between the "IOU this drawing", "nunc pro tunc" and the two dates, it is very likely that Jay was supposed to have drawn this for you at the party on 10/7 and wound up actually doing it on 10/10. "nunc pro tunc" is Latin for "now for then". So, he drew it "now" (on 10/10) to cover when he should have done it "then" (10/7).