My uncle's b-day is Dec 12,1935. But my grandfather went to the town hall in January and wrote his b-day as Jan 1st 1936. The main reason was, according to my uncle was that when he was of age to be drafted for the army he wolud be able to stay home a extra year. Also most of the children were born at home or with the help of a midwife, It was the husbands job to go to city hall to register the birth. Sometimes the man was in the country working, a week or weeks at a time such as the harvests of the grapes, olives, lemons, wheat etc. When ever he got there he would go register, They did not put a great deal of emphasis on the exact date.