Rashi to Shemos 18:13 starts by saying:
ויהי ממחרת: מוצאי יום הכיפורים היה
It came about on the next day: This was the day after Yom Kippur.
And then goes on to describe the first Yom Kippur right after the giving of the Torah which would have been the first available day to judge the people.
But then Rashi goes on to say:
ואין פרשה זו כתובה כסדר, שלא נאמר ויהי ממחרת עד שנה שניה
Hence, this section is not written in [chronological] order, for "It came about on the next day," was not said until the second year.
In fact, the Rashi in its entirety would make no sense other than to say that this was the first Yom Kippur, because by the second Yisro already left (Bamidbar Chapters 9 and 10).
So what is he saying it wasn't said until the second year?