Data Sources and Attribution

JewishCalendarTools.com is built on open source software and public astronomical standards. This page lists every data source and library the site depends on, along with its license, so you can verify our work independently.

Calculation Libraries

@hebcal/core (GPL-2.0)

The core Hebrew calendar engine is @hebcal/core version 6.9.2, licensed under the GNU General Public License version 2. It provides Hebrew date conversion, holiday schedules, Torah portion cycles, and Hebrew year structure (leap years, month lengths, and postponement rules). The library is a JavaScript port of the classic Hebrew calendar algorithms and is one of the most widely used open source Jewish calendar implementations.

@hebcal/cities (BSD-2-Clause)

City coordinates and time zones come from @hebcal/cities version 6.2.1, licensed under a permissive BSD 2-Clause license. It provides latitude, longitude, elevation, time zone, and the standard candle-lighting and Havdalah customs for major Jewish communities worldwide.

Astronomical Calculations

Sunset, nightfall, and related sun-position times are computed from the solar equations published by the United States National Oceanic and Atmospheric Administration (NOAA), including the equation of time and solar declination corrections. The same equations underlie nautical and rabbinic almanacs. The implementation applies your location’s latitude, longitude, elevation, and time zone offset, so results are specific to your selected city.

Time Zone Data

Time zone rules, including daylight saving transitions, follow the public IANA time zone database, which is maintained by the international tz community and distributed with every operating system.

Open Source Licensing Notice

The @hebcal/core library is distributed under GPL-2.0. In keeping with that license, the Hebrew calendar functionality bundled into this site’s calculators is offered under the same terms, and the source of the underlying library remains freely available from its maintainers. The @hebcal/cities database is used under its BSD 2-Clause license, which permits redistribution with attribution.

Verification References

Results from this site were cross-checked during development against published calendars from established providers, including Rosh Hashanah, Yom Kippur, and Pesach dates for multiple years, weekly Torah portions, and candle-lighting times for major cities in the United States, Canada, Israel, Europe, and Australia.

Limitations

Times shown are calculated, not observed. Local terrain, elevation differences within a city, and personal custom can shift practical times by a minute or more. During Israeli daylight saving transitions or at high latitudes in summer, verify with your community’s published schedule.