I want my retirement benefits to begin precisely on the date I turn 70 (i.e. - 5/21/47), what month then do I indicate I want the benefits to begin May or June ? Is there proration by the birth date within the month ?
You will want to claim benefits effective with May 2017. It doesn't matter what day of the month you were born, you will be paid for the full month and receive the maximum number of delayed retirement credits. Social Security benefits are paid in the month following the month for which they are due, so your payment for May will be paid in June.