This is from the 9th edition of Intermediate Accounting by authors: Spiceland, Nelson, and Thomas. There was no textbook solution to this question! Can you please answer question #1 and refund my question! What is Ensor's stock option measurement date?

On October 15, 2017, the board of directors of Ensor Materials Corporation approved a stock option plan for key executives. On January 1, 2018, 20 million stock options were granted, exercisable for 20 million shares of Ensor's $1 par common stock. The options are exercisable between January 1, 2021, and December 31, 2023, at 80% of the quoted market price on January 1, 2018, which was $15. The fair value of the 20 million options, estimated by an appropriate option pricing model, is $6 per option. Ensor chooses the option to recognize forfeitures only when they occur. Ten percent (2 million) of the options were forfeited when an executive resigned in 2019. All other options were exercised on July 12, 2022, when the stock's price jumped unexpectedly to $19 per share. Required: 1. When is Ensor's stock option measurement date