My friend and I have the same birthday. If our birthdays are the same, why are the Hebrew dates for our birthdays different? Her Hebrew birthday is after mine.
If you and your friend had literally been born on the same day, you would both celebrate your birthdays, secular or Hebrew, together every year.
But I can tell from what you write that even if you were both born on the same date, you were not born literally on the same day. Rather, if your secular birthdays and Hebrew birthdays do not coincide, you must have been born in different years, which makes for different dates in the Hebrew calendar. I’ll explain:
The secular or Gregorian calendar is solar-based. That’s why it’s comprised of 365 days, which is the time it takes the earth to revolve once around the sun. For this reason its months are in sync with the seasons of the year — spring, summer, fall and winter. But these months are not, as the word would suggest, connected to the moon. So while the secular calendar is in sync with the seasons, there is no relationship between its months and the various phases of the moon.
The Jewish calendar, however, is primarily lunar, where the beginning of the month occurs on the new moon, the middle of the month occurs on the full moon and the month concludes at the end of the waning moon. Since there are approximately 29.5 days in the lunar cycle, some Jewish months are 29 days and others are 30, but all average out and correspond to the twelve 29½-day moon-months, which result in a 354-day year.
This is the fundamental reason why dates of the Gregorian and Hebrew calendars do not coincide. There is a built-in discrepancy of 11 days between a 354-day lunar-based year and a 365-day solar-based year. Thus two people such as you and your friend who were born on the same secular date of different years will have been born on different Hebrew dates!
By the way, because of this 11-day difference, the months of a purely lunar calendar will lag behind the solar-based seasons by approximately one month every three years. And throughout the years, its months will drift throughout the entire spectrum of the seasons.
The Islamic calendar is such a purely lunar system, where, for example, the month-long, day-time fast of Ramadan is sometimes in the short, cool days of winter and other times in the long, hot days of summer.
So while the Gregorian calendar is in sync with the sun, its months are out of sync with the moon. Conversely, while the Islamic calendar’s months are in sync with the moon, they are totally out of sync with the sun and the seasons.
However, the earlier, ancient Jewish calendar ingeniously resolves the inherent tensions of the purely solar or lunar systems as adopted by the Gregorian or Islamic calendars. It does so by using a lunisolar calendar that simultaneously preserves true lunar months within the seasons of the solar year.
The way this works is that since a discrepancy of about one month occurs over three years, approximately every three years (specifically, 7 times in 19 years), a second month of Adar is added at the end of the winter in order to fulfill the Torah’s mandate for Nisan, the month of Redemption, to occur in the spring. The intercalation of this extra month ensures that all of the months and their respective holidays are always in balance with their relevant seasons.