20 CFR §218.5

1. (a)An annuity begins either on the earliest date permitted by law, or on a specific date chosen by the applicant. If the applicant chooses a specific date, that date must not be before the earliest date permitted by law.
2. (b)An annuity may not begin on the thirty-first day of a month, unless the claimant would lose benefits if the annuity begins on the first day of the following month. No annuity is payable for the thirty-first day of any month.