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Calculation Methodology

Every result on JewishCalendarTools.com is produced by the same calculation engine, and this page explains exactly how it works. We believe calendar tools should be transparent: you should never have to wonder where a time or a date came from. If a detail on this page seems unclear, contact us and we will expand it.

Everything Runs in Your Browser

All six tools are client-side calculators. The engine is a self-contained JavaScript module that downloads with the page and runs entirely on your device. Nothing you type, select, or calculate is ever transmitted to our servers or stored in a database. When you close the page, your entries are gone. Only one optional convenience exists: the Shabbat tool can save your chosen city on your own device through your browser’s local storage, and you can clear it at any time with the reset control.

Hebrew Calendar Conversion

The Hebrew calendar is lunisolar: months follow the lunar cycle while years stay aligned with the solar seasons. Conversion between Hebrew and civil dates is exact arithmetic, not approximation. Our engine uses the open source @hebcal/core library (version 6.9.2), a mature and widely used implementation of the classic Hebrew calendar algorithms, including:

  • Month lengths: 29 or 30 days per lunar month, following the fixed calendar established by Hillel II.
  • Leap years: 7 leap years in every 19-year cycle, each adding the month of Adar I before Adar II.
  • Year types: all 14 legal combinations of deficient, regular, and full years, which determine whether Cheshvan and Kislev have 29 or 30 days.
  • Dechiyot (postponement rules): the four rules that shift Rosh Hashanah to prevent certain weekday and time-of-week configurations.

Because the conversions follow the fixed rabbinic calendar, two visitors converting the same date anywhere in the world will always see the same Hebrew date, before any sunset adjustments.

Days Begin at Sunset

In Jewish law, a calendar day runs from sunset to sunset, not midnight to midnight. Every tool on this site applies this rule:

  • The birthday calculator asks whether a birth happened before or after sunset before assigning the Hebrew birth date.
  • The Yahrzeit calculator applies the same sunset boundary to the recorded date of passing.
  • The date converter shifts the resulting Hebrew date by one day when you indicate a time after sunset.

Sunset and Sun Position Mathematics

Sunset, candle-lighting, and Havdalah times depend on the sun’s position relative to the horizon at your exact location. Our engine calculates solar positions from the standard NOAA solar equations using your city’s latitude, longitude, elevation, and time zone, including adjustments for the equation of time. The results match published rabbinic and almanac times for the tested cities to within a minute under normal conditions.

Candle-Lighting Time

Candle lighting is a fixed number of minutes before sunset. The common Diaspora custom is 18 minutes. Many communities light earlier, so the Shabbat tool offers presets of 18, 20, 30, and 40 minutes plus a custom entry.

Jerusalem is a special case: by long-standing custom, candle lighting in Jerusalem is 40 minutes before sunset. When you choose Jerusalem in the Shabbat tool, the 40-minute preset is selected automatically, and you can change it if your community follows a different custom.

Havdalah and Nightfall

Havdalah is recited after nightfall, defined as the moment three medium stars are visible. Astronomically, this is expressed as degrees of solar depression below the horizon. Our default is 8.5 degrees, the widely used Rabbeinu Tam-associated nightfall used by many calendars. Alternatives of 42, 50, and 72 minutes after sunset are offered for communities that follow those customs.

High Latitude Handling

At high latitudes around the summer solstice, the sun may never reach the required depression angle, so fixed-degree nightfall cannot be computed. In those cases the tool reports the situation explicitly rather than printing a misleading time. For extreme latitudes, follow your community’s rabbinic guidance, since customs vary substantially.

Torah Portions and Holidays

Weekly Torah portions follow the standard annual cycle, which differs between Israel and the Diaspora in weeks when a festival falls immediately after Shabbat. When you choose a region, the correct schedule is applied, and the holiday calendar reflects the same choice, including the difference in Shemini Atzeret and Simchat Torah observance.

Verification

The engine is tested against a suite of real-world verification fixtures, including Rosh Hashanah 5787, Yom Kippur and Pesach dates, leap-year Adar I and Adar II placements, a 30 Kislev Yahrzeit, and published candle-lighting times for major cities. Every release must pass the full suite in multiple time zones before it is deployed.

When Calculations Are Not Enough

A calculator can produce a date or a time, but it cannot rule on halacha. When customs differ, when a family tradition conflicts with a printed result, or when an edge case such as a burial delay is involved, the tool states the range of practice and advises consulting a qualified rabbi. That is the correct division of labor, and we keep it that way.