In some cases, only month and year are allowed for birthdate. In most cases, you can simply enter the month and year the person was born. However, for individuals born on the first of the month, you should enter the prior month. For individuals born on January 1, you should enter December of the prior year.
This is because Social Security considers individuals born on the first of the month to have attained their new age on the prior day, i.e. if you turn 70 on your birthday of October 1, Social Security considers you to have attained age 70 in September.