How Hebrew Birthdays Work
Understand sunset boundaries, Hebrew leap years, Adar I and Adar II, and why dedicated calendar conversion is necessary.
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The Hebrew Date Begins at Sunset
A civil date alone may be insufficient. A birth after sunset belongs to the next Hebrew date, so time and location can matter.
Leap Years and Adar
The fixed Hebrew calendar uses a 19-year cycle with leap years that contain Adar I and Adar II. Ordinary years contain one Adar.
Birthday Rules
The correct anniversary treatment can depend on whether the birth was in ordinary Adar, Adar I, or Adar II. Religious customs and legal contexts may apply different rules.
Calculator Scope
The Hebrew Birthday Calculator calculates Hebrew age and exposes Adar review. It deliberately does not claim to perform a full Gregorian conversion with unsupported generic formula functions.
Full Technical Implementation
Hebcal provides maintained Hebrew/Gregorian conversion libraries and birthday or anniversary functions. Its hdate implementation uses Rata Die day numbers internally.
Historical Dates
Projected ancient dates can be mathematically consistent without being historically exact because the fixed calendar developed over time.
Related Guides
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Frequently Asked Questions
- Why does sunset matter?
- Jewish calendar days begin at sunset rather than civil midnight.
- Does every Hebrew year have Adar II?
- No. Adar II occurs only in Hebrew leap years.
- Can a generic date formula safely replace Hebcal?
- Not without implementing and testing the complete Hebrew calendar rules.
Last updated 7/20/2026