Some birthdays appear on different dates because the celebration follows a lunar or lunisolar calendar, because a person was born on leap day, or because family tradition uses an observed date. The birth date itself does not change; the calendar used to mark it may.
Leap-day birthdays
A person born on 29 February sees that date only in leap years; NIST explains why the extra calendar day is added. In other years, they may celebrate on 28 February or 1 March according to preference, family practice or local legal rules.
Lunar and lunisolar birthdays
Calendars that follow Moon cycles do not line up with the Gregorian calendar in the same way every year. A birthday kept by that calendar can therefore move across Gregorian dates. Practices differ across cultures and families.
Time zones and historical records
A birth near midnight can be recorded on different civil dates depending on place and time zone. Older records may also use a calendar system different from today’s local standard.
Store the meaning clearly
Ask which date the person observes. Save the preferred recurring date and note the calendar tradition if useful. A custom date type can preserve context without forcing every birthday into one rule.
Questions people ask
When do leap-day babies celebrate?
They choose 28 February or 1 March in common practice; legal rules can vary by place and purpose.
Does a lunar birthday move randomly?
No. It follows its own calendar rules, which map to different Gregorian dates.
