2)Two boys were born on the same day, the same year.
One becomes Bar Mitzvah a day earlier than the other.
1) The Mishna Verurah (55 sk 45) writes If someone was born on Rosh Chodesh Kislev that had only
one day Rosh Chodesh and thirteen years later there are two days Rosh Chodesh (this year ) ,he
becomes Bar Mitzvah on the 30th day in Cheshvan. (not on his birthday 1st. Kislev)
2) The Piskei Teshuvos brings a Shita that holds, it depends if the child was born before the Molad or
after the Molad. Hence two children born on the same day, becoming Bar Mitzvah one day apart.(this year)