Jewish law has measured time by the sun since the giving of the Torah at Sinai. The halachic clock displayed on DaatTime.Today anchors 12:00:00 to sunrise and 00:00:00 to sunset, with the 12 halachic hours of the day filling the space between them proportionally.

This is not an invention of modernity. The Talmud (Berakhot 9b) discusses prayer times in terms of proportional hours, and the Mishnah (Berakhot 1:1) measures obligations from nightfall to dawn and dawn to nightfall — the sun is the clock.

This dispute has enormous practical impact. On a summer day in Jerusalem, the MGA shaah zmanit can be significantly longer than the GRA shaah zmanit, meaning the MGA deadline for Shema arrives earlier on the clock.

The Talmud in Pesachim 94a states that the time from Alot HaShachar to sunrise equals the time to walk 4 mil. According to the Rambam (Hilchot Shabbat, commentary on Mishnah Berakhot 1:1), one mil = 18 minutes, therefore Alot = 72 minutes before sunrise.

The GRA and Rishonim translate this into degrees: at Jerusalem's latitude on the equinox, 72 minutes before sunrise corresponds to the sun being 16.1° below the horizon. This degree value is then used universally for all locations and all dates, making Alot location-aware rather than a fixed 72 minutes.

The Mishnah in Berakhot 1:2 establishes misheyakir as the time when there is enough light to distinguish between the blue (techelet) and white threads of the tzitzit, and to recognize a familiar acquaintance from 4 cubits away.

Unlike Alot which has a clear Talmudic time-based definition, misheyakir has no fixed degree in the Talmud — it is entirely observational. Modern poskim translate this to approximately 10.2° to 11.5° below the horizon, with 10.5° as a widely accepted middle position used by KosherJava and most digital zmanim calculators.

Berakhot 9b and 26a establish sunrise as the preferred time for prayer — one who begins the Amidah at sunrise and finishes it as the sun fully rises is called a vatikin (pious one). The Talmud describes this as the ideal fulfillment of "They shall fear You with the sun" (Tehillim 72:5).

On the halachic clock, all proportional calculations anchor here: Shema is due 3 hours after this point (GRA), Chatzos is 6 hours after, and all afternoon times are measured from this sunrise baseline.

The Torah says Shema is recited "when you rise up" (Devarim 6:7). The Talmud in Berakhot 9b-10a interprets this as the time when "kings arise" — the 3rd hour of the day — since the obligation mimics the time when most people begin their day.

Chatzos is the halachic pivot of the day. It is the absolute latest time for Shacharit (ex post facto), the dividing line between morning and afternoon prohibitions, and the anchor for all Mincha calculations. On fast days, half-day personal fasts end at Chatzos.

The app calculates Chatzos using the NOAA solar transit algorithm — the precise moment the sun crosses the local meridian — making it astronomically exact to the second for any location on earth.

The three Mincha times correspond to the afternoon daily offering (Tamid) in the Temple. Mincha Gedola is half an hour after Chatzos (the earliest the afternoon Tamid was brought). Mincha Ketana reflects the preferred time. Plag HaMincha is 1.25 halachic hours before sunset — the point at which one may already treat the next day as having begun for purposes of accepting Shabbat.

The Shulchan Aruch (Orach Chaim 261:2) rules that one must add time before Shabbat. The 18-minute standard became widespread in Diaspora communities. Jerusalem's 40-minute custom reflects the ruling of Rav Yosef Chaim Sonnenfeld and the Yerushalmi Luach, based on the stricter local tradition to begin Shabbat earlier.

DaatTime.Today automatically detects Jerusalem (within ~8km of city center) and switches to 40 minutes. All other locations use 18 minutes. This row only appears on Erev Shabbat and Erev Yom Tov.

Shabbat 34b establishes that sunset marks the beginning of the ambiguous twilight period (bein hashmashot) — no longer clearly day, not yet clearly night. Melacha (forbidden labor) on Shabbat must cease at this point. All daytime mitzvot must be completed before sunset.

Shabbat 35a describes nightfall as the appearance of three stars. The size of the stars matters — three medium stars (not small) according to the Shulchan Aruch. Modern astronomical research establishes that medium stars become visible when the sun is approximately 7.083° below the horizon, while small (faint) stars appear at approximately 8.5°. The widespread practice follows 8.5° as the standard for Tzais.

Rabbeinu Tam (Rabbi Yaakov ben Meir, 1100–1171) reconciled two contradictory Talmudic passages about twilight by positing two sunsets: the first when the sun crosses the horizon (what we see), and the second — the halachic sunset — occurring 58.5 minutes later (then nightfall at 72 minutes). This positions Tzais at a fixed 72 minutes after the visible sunset.

Most Ashkenazic and Sephardic authorities today follow the Geonim's position (8.5°), which gives an earlier Tzais. Chassidic communities, particularly Chabad, maintain the Rabbeinu Tam 72-minute standard for Havdalah, often performing Havdalah at both times — Tzais for the de-facto acceptance, Rabbeinu Tam for the formal Havdalah.

All zmanim calculations are performed by the Python zmanim library, a port of the KosherJava project by Eliyahu Hershfeld, using the NOAA solar algorithm based on Astronomical Algorithms by Jean Meeus. Timezone detection uses the timezonefinder library — fully offline, no external API.