Hebcal for UNIX is a free program and is available for download at https://github.com/hebcal/hebcal. This is the same engine that powers the hebcal.com website. The downloadable program has a text (command-line) interface, not a fancy graphical one.
Kaluach is an unrelated program available at http://www.kaluach.com/. The author requests donations, but does not have a set price.
Shabbat begins 18 minutes before sundown on Friday night. In Jerusalem, Shabbat begins 40 minutes before sundown.
According to the United States Naval Observatory,
Sunrise or sunset is defined to occur when the geometric zenith distance of center of the Sun is 90.8333 degrees. That is, the center of the Sun is geometrically 50 arcminutes below a horizontal plane. For an observer at sea level with a level, unobstructed horizon, under average atmospheric conditions, the upper limb of the Sun will then appear to be tangent to the horizon. The 50-arcminute geometric depression of the Sun’s center used for the computations is obtained by adding the average apparent radius of the Sun (16 arcminutes) to the average amount of atmospheric refraction at the horizon (34 arcminutes). [USNO]
Shabbat ends after sundown on Saturday night when there are three stars visible. Depending on latitude and longitude, this is usually between 42 and 72 minutes after sundown.
According to Wikipedia,
There are three widely observed practices, all of which have support in the halachic literature:
- Appearance of three medium-sized stars in the sky (sun 7°5′ below the horizon, or 42 minutes after sundown), as in the Talmud. This is normative practice in Conservative Judaism. In Orthodox Judaism, this position is used widely for the end of rabbinical fasts, but less frequently for the end of Shabbat or biblical festivals.
- Appearance of three small stars widely spaced in the sky (sun 8.5°-8.75° below the horizon): common practice in much of Orthodox Judaism [10]
- “50 minutes after sundown” is actually a variant of this position. Rabbi Moshe Feinstein ruled this way because most people cannot easily calculate when “8.5° after sundown” will occur, and 8.5° takes 50 minutes at its longest, near the summer solstice, at the latitude of much of the United States.[2]
- 72 minutes after sundown (“opinion of Rabbeinu Tam“): equivalent to other definitions of nightfall, and safe according to all opinions. Common practice in Chasidic and other Charedi communities
Shabbat begins 18 minutes before sundown on Friday night.
Havdalah is 72 minutes after sundown on Saturday night.
So in a given week, Havdalah time is typically 90 minutes after the previous day’s candle-lighting time (18 + 72 = 90). Sometimes there’s an extra minute or two difference, and that’s due to sunset actually differing by a minute or two between Friday and Saturday nights.
Since candle lighting times are determined from latitude and longitude, a process called geocoding is used to determine the geographic position of a zip code.
Our zip code database comes from the 1999 ZIP Code file from the U.S. Census Bureau’s Gazetteer project.
The Gazetteer data from 1999 is much more complete than their data from 1990 — but still not as good as the commercial-quality data you get from the U.S. Post Office. Gazetteer contains zip code, city name, latitude and lognitude but is missing time zone.
Fortunately, the Gazetteer data contain a FIPS code (which represents the county a zipcode is part of). The U.S. National Weather Service provides county-timezone data (keyed on FIPS code), which contains timezone and Daylight Saving Time data for each county. By joining these two data sources, we can get a complete picture of each zip code.