At that time I said to you, ‘I am not able to bear you by myself.'
Deuteronomy 1:9

This recounting of the appointing of judges as recommended by Jethro appears to place the event almost 40 years after Exodus 18 says it happened, but that's a misunderstanding of the nature of the text. "At that time" refers to the era of Sinai, as in "during the time we spent in the wilderness around Sinai", not to a precise sequence of events.

Interesting aside: It appears that the people nominated their own leaders who were then confirmed or ranked by Moses.