Thanks for the reply. You're right, I hadn't completed the application. Further on in the application the applicant can specify the date on which benefits should start. That answered my question.

I started my application for SS benefits on Jan 31st at age 69 with the intention of submitting it later in the year at age 70. On exiting the process before the application was finished I got this instruction: "We may use 01/31/2021 as the official ...