An interesting question arises in the case of a boy who was born during a leap year, on the 30th day of Adar Rishon (the first Adar), and the year of his Bar-Misva is a regular, twelve-month year, when there is only one month of Adar. Since there is no 30th day of Adar in a regular year, it is unclear which day is considered his birthday when he becomes a Bar-Misva.
According to some Poskim, he becomes a Bar-Misva on the 30th of Shevat. Since the boy was born on the day preceding the first of Adar Sheni – the second Adar, which is considered the real month of Adar during a leap year – he turns thirteen on the day preceding the first of Adar – namely, the 30th of Shevat. This is the view of the Binyan Sion (Rav Yaakob Ettlinger, Germany, 1798-1871), the Minhat Yishak (Rav Yishak Weiss, 1902-1989), and Hacham David Yosef, in Halacha Berura.
However, Rav Shlomo Zalman Auerbach (Jerusalem, 1910-1995) maintained that it cannot be determined conclusively whether this child becomes a Bar-Misva on 30 Shevat or on the first of Nissan – the equivalent of 30 Adar, since Adar has only 29 days. Therefore, the boy must take into account both possibilities, meaning, he must strictly observe all Misvot already from 30 Shevat, but he cannot be counted toward a Minyan until the first of Nissan.
A somewhat similar question arises in the case of a boy born on 30 Shevat in a regular year, and his Bar-Misva year is a leap year. The Binyan Sion ruled that since he was born on the day before the first of Adar, he becomes a Bar-Misva on 30 Adar Rishon – the day before the first of the real month of Adar, Adar Sheni. Others, however, maintain that since he was born during the month of Shevat, his Bar-Misva date is not affected at all by the extra Adar added in the year of his Bar-Misva, and thus he becomes Bar-Misva on 30 Shevat. Here, too, Rav Shlomo Zalman ruled that the boy must follow the stringencies of both opinions. He must strictly observe all the Misvot already from 30 Shevat, but he may not be counted toward a Minyan until 30 Adar Rishon.