Zmanim explained and their sources

When the sages say "until three hours" or "until four hours", they do not mean clock hours. The day, whether long or short, is divided into twelve equal parts, each one an hour. A longer chosen interval produces a longer proportional hour; a shorter interval produces a shorter one. A proportional hour equals 60 minutes only when that interval lasts exactly 12 clock hours.

This is how the times for Shema, the morning prayer, Mincha and plag are calculated. Here, "day" means the interval used in that calculation, rather than the civil day beginning at midnight.

Sources: Shulchan Aruch, Orach Chaim 233:1, in the Rema’s note, citing the Rambam; Mishnah Berurah 233:4.

Two common bases are dawn to nightfall and sunrise to sunset. They explain the families of times shown in the app.

For Magen Avraham, hours run from dawn to the appearance of the stars. For the Vilna Gaon (Gra), they run from sunrise to sunset. The definitions of dawn and nightfall must also be chosen. Saying "MGA" alone does not identify every calculation parameter.

In the app, the "Gra" family uses ordinary sunrise and sunset on level terrain. The "Chabad" family uses "true" sunrise and sunset, as the next topic explains.

The app's MGA variants use 72 fixed minutes or angles of 16.1° and 19.8°, labeled 72 and 90 minutes in degrees.

Sources: Magen Avraham 58:1; Biur HaGra, Orach Chaim 459:2; Mishnah Berurah 58:4 and 233:4, recording both opinions.

In Shulchan Aruch HaRav, the Baal HaTanya counts the Shema deadline from dawn. In the laws of Passover and later in the Siddur, he counts the day from sunrise to sunset. Chabad practice follows the later Siddur.

In the Siddur he distinguishes two moments of sunset. Visible sunset is when light leaves the treetops and rooftops; "true" sunset follows about 4 minutes later, when light leaves the mountaintops of Eretz Israel. Shabbat candles are lit before visible sunset. The Gemara already uses Mount Carmel as a measure of twilight.

Today's Chabad calendar, as published by Chabad.org, converts this into angles: proportional hours run between "true" sunrise and sunset, with the Sun's center 1.583° below the horizon; dawn is at 16.9° and three medium stars at 6°. This is the app's "Chabad" family. These figures come from Chabad.org; some Chabad rabbis count hours using ordinary sunrise and sunset, as in the "Gra" family.

R. Avraham Chaim Naeh, founder of the Kolel Chabad calendar, placed dawn two hours before sunrise in Eretz Israel at the equinox. The app shows that solar position, 26°, in the "R. Naeh" row.

Sources: Shulchan Aruch HaRav, Orach Chaim 58:3 and 443:4; Piskei HaSiddur, Laws of Shema 18; Seder Hachnasat Shabbat 2; Shabbat 35a.

The Talmud measures dawn and nightfall by walking time. According to Rabbi Yehudah, one walks four mil from dawn to sunrise and another four from sunset to the appearance of the stars.

The duration of a mil is disputed. The Shulchan Aruch gives a quarter hour plus a twentieth of an hour: 18 minutes, making four mil 72. The Gra calculates three eighths of an hour: 22.5 minutes, making four mil 90. The Gra also cites the Rambam's calculation of two fifths of an hour: 24 minutes.

Three quarters of these measures are 13.5, 16.875 and 18 minutes. In the app, these numbers are references for rows calculated in degrees; they are not fixed minutes added to sunset in every location or season.

Sources: Pesachim 94a; Shulchan Aruch, Orach Chaim 459:2; Biur HaGra, Orach Chaim 459:2.

Dawn is a reference for several daytime obligations. For the Shacharit Amidah, the Shulchan Aruch distinguishes the ideal time, at sunrise, from a prayer already valid when recited after dawn and the brightening of the east. Validity when praying early does not make dawn the ideal prayer time.

The app shows 72 clock minutes before sunrise and angular measures: 16.1° and 19.8° below the horizon, 16.9° in the "Chabad" family and 26° in the "R. Naeh" row. For angular rows, the interval until sunrise varies with location and date.

Sources: Pesachim 94a; Shulchan Aruch, Orach Chaim 89:1.

The Mishnah marks the start of morning Shema by light: when blue can be distinguished from white. The Shulchan Aruch gives a practical criterion: recognizing, at four cubits, an acquaintance with whom one has some familiarity. This criterion also applies to putting on tefillin. For the tallit blessing, the source discusses distinguishing blue from white; early use has its own details and customs.

These sources describe recognition and light levels. The angles in the table are published methods for applying that criterion: 11.5° is associated with the custom of Eretz Israel, 11° with R. Meir Posen's guidance and 10.2° with Otzrot Yerushalayim. The app offers all three.

Sources: Mishnah Berakhot 1:2; Shulchan Aruch, Orach Chaim 30:1, 18:3 and 58:1.

This is the ideal time for the morning prayer. The vatikin, the especially diligent, finished Shema and its blessings precisely at sunrise and began the Amidah immediately afterward.

The app calculates when the Sun's upper edge appears over a level horizon, including atmospheric refraction. If you enter an elevation with a lower horizon, it also shows an estimate for that profile. The calculation does not reconstruct actual mountains, buildings, temperature or pressure. Proportional hours continue to use the events on level terrain.

Sources: Shulchan Aruch, Orach Chaim 58:1 and 89:1.

In the Mishnah, Rabbi Yehoshua allows morning Shema until three hours because princes rise at that time. The Shulchan Aruch rules accordingly: until the end of the third hour, a quarter of the day.

The app adds three proportional hours to the start of the chosen interval. This is the deadline for Shema in its proper time, not the Amidah deadline. Missing it does not mean abandoning Shema: the Shulchan Aruch distinguishes later reading and the deadline for its blessings.

Sources: Mishnah Berakhot 1:2; Shulchan Aruch, Orach Chaim 58:1 and 58:6.

In the Mishnah, the sages allow the morning prayer until midday, and Rabbi Yehudah until four hours. The Shulchan Aruch follows Rabbi Yehudah: until the end of the fourth hour, a third of the day. For someone who is late, the source distinguishes prayer until midday, already outside its proper time. After chatzot, one does not simply pray Shacharit as before: in cases of error or unavoidable circumstances, rules for making up prayer, called tashlumin, apply alongside the next prayer.

Sources: Mishnah Berakhot 4:1; Shulchan Aruch, Orach Chaim 89:1 and 108:1.

Chatzot marks the middle of the day or night. Daytime chatzot appears in the late limit for Shacharit and in Mincha calculations. It does not necessarily coincide with 12:00 on the clock.

The app uses the midpoint between level-horizon sunrise and sunset for midday, and the midpoint between level-horizon sunset and the next sunrise for midnight. These are this calculator's conventions. The daytime midpoint and solar transit are documented methods with close but distinct results. Transit is the Sun crossing the local meridian; the midpoint divides the sunrise-to-sunset interval into two equal parts.

For midnight, Shulchan Aruch HaRav, second edition, 1:8 gives another explicit rule: 12 clock hours after midday. That calculation may differ from the nighttime midpoint used by the app. When using chatzot as a mitzvah deadline, check which method you follow.

Chatzot in prayer: Shulchan Aruch, Orach Chaim 89:1. Midnight: Shulchan Aruch HaRav, second edition, 1:8.

The Shulchan Aruch places Mincha gedolah at six and a half hours and ketanah at nine and a half. These are proportional hours: the "half hour" does not always mean 30 clock minutes. The source presents ketanah as the main time; in some circumstances one prays earlier.

For gedolah, the app offers chatzot plus half a proportional hour, chatzot plus 30 clock minutes, and "lechumra", the later of the two. MGA rows use that variant's own interval. Ketanah marks the start of the preferred period, rather than a deadline for finishing Mincha.

Source: Shulchan Aruch, Orach Chaim 233:1.

In the Mishnah, the sages allow Mincha until night, and Rabbi Yehudah until plag hamincha, which the Shulchan Aruch defines as the end of eleven hours minus a quarter: 10.75 proportional hours. Applying the two opinions requires consistency between Mincha and Arvit. As a general rule, someone who advances the Arvit Amidah to after plag should not combine that with Mincha after the same plag; the source also discusses exceptions and situations of need.

Plag is also a reference for accepting Shabbat early. This does not automatically advance the time for evening Shema: when recited before its time, it must be repeated later.

Without seconds, the app rounds the Gra and Magen Avraham plag down, the safe side for those using it as a Mincha deadline; those accepting Shabbat early from this plag should use the following minute. "Chabad" plag rounds up, as on Chabad.org: those using it as a Mincha deadline should use the preceding minute (if it shows 16:56, finish by 16:55). Seconds can always be enabled in Settings.

Sources: Mishnah Berakhot 4:1; Shulchan Aruch, Orach Chaim 233:1 and 261:2, in the Rema’s note; 235:1 for evening Shema.

Some weekday time is added to Shabbat. The Mishnah Berurah asks that lighting be completed before sunset, without waiting until the last moment. Candle-lighting time anticipates the start of Shabbat; it is neither sunset nor the end of Shabbat.

The advance depends on local custom. The app defaults to 18 minutes, or 40 when the selected city is Jerusalem; other options appear in Settings. It subtracts these minutes from the displayed sunset, including the lower-horizon estimate when provided.

Sources: Shabbat 35b; Shulchan Aruch, Orach Chaim 261:2; Mishnah Berurah 261:23.

On Yom Tov other than Shabbat or Yom Kippur, candles are sometimes lit at night from an existing flame. When passing from Shabbat to Yom Tov and on the second night of Yom Tov, one waits until nightfall; if the incoming night is Shabbat, lighting must take place before sunset.

The app's candle row covers lighting before sunset. Its absence when passing from one holy day to another does not mean candles are unnecessary. The app also does not show an "end" between consecutive holy days.

Between day and night lies a period of doubt, bein hashmashot, measured by one opinion discussed in the Talmud as three quarters of a mil. Its starting point is a major dispute.

Rabbeinu Tam distinguishes the beginning and end of shkiah to reconcile the passages about four mil and three quarters of a mil. In this reading, the four-mil interval includes three and a quarter mil before the final twilight. With an 18-minute mil, four mil are 72 minutes. The Shulchan Aruch presents this structure, but applying it to a different location or season also requires choosing a calculation method.

The Mishnah Berurah records that many early commentators, followed by the Gra, begin twilight when the Sun disappears. This does not make the 13.5-to-18-minute rows a general authorization to end Shabbat: the app calculates those rows in degrees, and the criterion for ending Shabbat is treated separately.

Sources: Shabbat 34b–35a; Tosafot, Pesachim 94a; Shulchan Aruch, Orach Chaim 261:2; Mishnah Berurah 261:20 and 261:23.

One star: still day; two: twilight; three: night. Rabbi Yossi clarifies that these are medium stars, neither large ones visible by day nor small ones appearing only later at night.

For the end of Shabbat, the Shulchan Aruch requires more: three small stars close together. On a cloudy day, one waits until no doubt remains.

For the end of Shabbat and festivals, the app uses the Sun at 8.5° below the horizon. The angle is a calculation method, not a number given by the Talmud. Do not confuse this criterion with the earlier twilight or fast-ending rows.

In the Chabad preset, the main list distinguishes "Nightfall · Chabad, medium stars (6°)" from "End of Shabbat" or "End of Yom Tov · 3 small stars (8.5°)". The latter row appears only on the last day of a sequence of Shabbat and festivals, accounting for Israel and the diaspora. In the full list, the small-stars time remains available on every day it occurs.

The 6° "Chabad" row follows Chabad.org's published angle for three medium stars, about 24 minutes after sunset in Eretz Israel on the reference dates. Other conversions for medium stars reach 5°50′. Small stars are a different, later criterion: according to the Siddur, they appear only after a third of an hour following "true" sunset.

Sources: Shabbat 35b; Shulchan Aruch, Orach Chaim 293:2; Piskei HaSidur, Seder Hachnasat Shabbat 3.

Daytime fasts begin at dawn; the app's reminder uses 16.1°. For stopping eating, the reminder rounds this limit down; the dawn row rounds up because it represents a starting time. Eating before dawn after sleeping has its own conditions, described in Orach Chaim 564:1. Tisha B'Av also includes the night and begins before twilight starts.

On Yom Kippur, time must be added before the holy day begins. Sunset, used as a reference in the app's reminder, is not a time until which one may keep eating: one must stop earlier, according to the guidance and custom followed.

The end of daytime fasts is associated with the appearance of medium stars. The app's R. Tucazinsky and R. Feinstein rows use 6.5° and 7.1°, respectively. Do not automatically apply these rows to Yom Kippur or the end of Shabbat.

The app's fast-ending reminders use 8.5°, even when the table offers other rows. Choosing an earlier row requires identifying the situation and applicable guidance.

Sources: Shulchan Aruch, OC 564:1, 562:1, 553:2 and 608:1.

Counting begins on the night of 16 Nisan and continues for 49 days, counting days and weeks. The Shulchan Aruch recommends waiting for the stars to appear. Advancing Arvit does not automatically advance the count.

The app's reminder is issued at 8.5° and announces the count for the night beginning. The header's number follows the daytime portion of the selected date. Thus, at nightfall, the reminder announces the next count.

Someone who forgot to count at night should count by day without a blessing; missing a whole day has other consequences for subsequent blessings. The app does not record whether you counted or decide that situation.

Sources: Shulchan Aruch, OC 489:1, 489:2, 489:7 and 489:8.

The Talmud's measures were given for a particular place and season. The Mishnah Berurah writes that three quarters of a mil apply to the horizon of Babylonia in Nisan and Tishrei, when day and night are equal, and that twilight becomes much longer farther north. The Gra says the same of dawn: the interval until sunrise is not identical at every horizon or season.

There are therefore two ways to use a number such as 72. "Fixed minutes" adds or subtracts 72 clock minutes anywhere. "In degrees" asks how far below the horizon the Sun is 72 minutes before sunrise at the equinox in Jerusalem (16.1°), then finds when it reaches the same position at your location and date. The measure thus follows the sky's light rather than the clock.

16.1° and 19.8° are established values in calendar literature, adopted by the app. Conversion involves reference choices and rounding. The astronomical equinox and the day with exactly 12 hours between sunrise and sunset do not coincide because of the Sun's apparent radius and refraction; published methods use either reference. This explains some small differences between tables.

The two night measures can also switch order: at certain latitudes, "72 fixed minutes" occurs before 8.5°. The app keeps the fixed addition in that row; it does not guarantee that the app's three-star criterion has already been met.

Sources: Mishnah Berurah 261:23; Biur HaGra, Orach Chaim 459:2.

Without seconds, the app rounds starts up and deadlines down. For example, Mincha ketanah at 15:33:33 appears as 15:34; the Shema deadline at 08:44:59 appears as 08:44. Plag is treated as a Mincha deadline. "Chabad" rows follow Chabad.org's rounding to match its calendar: dawn down and plag up. This rounding is a display margin, not a resolution of differing opinions.

Alerts use the same conservative minutes. Midday and midnight are reference times displayed with seconds removed; no single rounding direction suits every religious use.

Actual sunrise and sunset depend on terrain, your elevation and atmospheric conditions, which no calculation fully knows. Do not rely on the times until the very last moment.

Someone praying outside Eretz Israel faces Eretz Israel and also directs their thoughts toward Jerusalem, the Temple and the Holy of Holies; in Eretz Israel one faces Jerusalem; in Jerusalem, the Temple. The Rema explains that, living west of Eretz Israel, we customarily pray facing east. Someone unable to determine directions turns their heart to their Father in heaven.

On phones, the location section shows the direction of Jerusalem along the shortest path on the globe to the Temple Mount. In the Americas it points northeast, rather than exactly east. Some prefer the constant map bearing, closer to east; when in doubt, consult your rabbi.

Sources: Shulchan Aruch, Orach Chaim 94:1, 94:2 with the Rema’s note, and 94:3.

Times belong to the selected civil date and appear in the location's time zone. The Hebrew date in the header corresponds to that civil date's daytime portion; the header does not change at nightfall. For the night beginning, consider the following Hebrew date. The calendar distinguishes Israel and the diaspora by the selected location's country.

At high latitudes, the Sun may not rise, set or reach an angle on a given date. "Does not occur" reports the absence of that astronomical event. It does not remove a religious obligation: the app does not choose a substitute rule for those regions.

Date conversion uses the local civil calendar. It does not resolve the halachic discussion about the International Date Line; in affected regions, identifying the religious day requires specific guidance.