Instructions: Tax Accounting

The client sent me the below email. Please review and draft me a response that I can copy and paste. Make sure you cite some authority in your response. Check out https://www.irs.gov/publications.

Hello-

My boss, Mr Shirley, finally recognized the value I bring to the company. He decided on December 25 to give me a bonus, 20% more than I got last year! The delivery driver brought the check on 12/28/18 but I didn't have time to cash it until 1/3/19 thanks to my out-of-town relatives. What year would this income get picked-up?

Thanks-

Clark Griswold