How are SPX index option prices determined in the 15-minute period after the close?

External question — Personal Finance Stack Exchange Author: 7529 Original post: https://money.stackexchange.com/questions/151538 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. There are SPX index options that expire every single day now (Mondays, Tuesdays, Wednesdays, Thursdays, and Fridays). I know that SPX options will trade until 4:00pm ET on the day of expiration. However, on days before the expiration day, they will trade until 4:15pm ET. On days before the expiration day, how are prices set for SPX options between 4:00pm and 4:15pm when the value of the underlying SPX index doesn't change between 4:00 and 4:15?