Zmanim (halachic times of day) calculation

By default we follow chabad.org¹ zmanim standards. That is, we follow the Alter Rebbe (aka Baal HaTanya) for times of day unless you choose otherwise.

The time for end of Shabbat and Yom Tov is shown according to the time for 3 small stars. On weekdays, however, we use the universal standard for nightfall of three medium stars. The Zmanim page will list both times on Shabbat and Yom Tov. Note that the later time must be observed for activities that are forbidden on Shabbat. See below for the methodology details.

Specifying the opinion you follow

The Zmanim page lets you specify which halachic opinion to use for calculating the daytime hours (sha'ah zmanit). This choice affects most daytime zmanim: latest time for Shema, latest time for Shacharit, Mincha Gedola, Mincha Ketana, and Plag HaMincha. Three general opinions are available:

Alter Rebbe (default) — Follows R' Shneur Zalman of Liadi in his Siddur². Halachic hours are measured from the amiti (astronomical) sunrise to amiti sunset — the moment the geometric center of the sun's disk crosses the horizon, ignoring refraction. End of Shabbat is set to when the sky darkens to 8.5° below the horizon. This is the standard for the Chabad community.

GR"A (Vilna Gaon) — Halachic hours are measured from visible sunrise (when the sun's upper edge first appears) to visible sunset. End of Shabbat is set to when three medium-sized stars are visible, approximately 6° below the horizon. This is the majority opinion followed by most Ashkenazi and Sephardi communities. See Biur HaGr"a OC 459:2³.

Magen Avraham — Halachic hours are measured from dawn (72 minutes before sunrise, or when the sun is 16.1° below the horizon) to nightfall (72 minutes after sunset). Because the "day" is longer, halachic hours are longer, and every daytime zman falls earlier than with the GR"A. Some communities follow this stringently for Shema. At very high latitudes where the sun never reaches 16.1° below the horizon, halachic hours are computed from halachic midnight instead — see Magen Avraham at high latitudes.

Note: Magen Avraham's version of the 72-minute nightfall is used only for calculating the halachic hour and the parts of the day; it is not used for any other halachic purpose⁴.

Additional zmanim (opt-in)

These reference rows are hidden by default and can be turned on in the gear menu:

• Dawn — Rav Avraham Chaim Naeh (−26°)
• Dawn — Sefer Bein Hashmashot (−19.8°)
• Misheyakir Sefer Eretz Israel and Kaf haChaim (−11.5°)
• Misheyakir — Sefer Nivreshet (−11.8°)
• Nightfall — Rabbeinu Tam (72 minutes after sunset)
• Nightfall — Sefer Bein Hashmashot / Igrot Moshe (−8.5°)
• Nightfall — Melamed Lehoil (−7.083°)

Zmanim definitions

Times are computed from your geographic location's astronomical events. Angles given below refer to the sun's depression below the horizon.

Sunrise and sunset

Visible sunrise (hanetz hachama) — The moment the top of the sun first appears above the horizon: sun's upper edge at −0.833° (accounting for average atmospheric refraction and the sun's semi-diameter).

Visible sunset (shkiat hachama) — Same 0.833° threshold at the end of the day: sun's upper edge disappearing on the western horizon.

Amiti sunrise / amiti sunset (Alter Rebbe) — The astronomical event: the geometric center of the sun's disk at −1.583°, i.e., disregarding refraction and taking the true horizon. This is what the Alter Rebbe treats as sunrise/sunset for the purpose of the halachic day. See Seder Hachnasat Shabbat, Siddur Torah Ohr².

Halachic hour (sha'ah zmanit)

A halachic hour is one-twelfth of the halachic day. What counts as the "day" depends on the opinion followed:

• GR"A — visible sunrise to visible sunset. See Biur HaGr"a OC 459:2³.
• Magen Avraham — dawn (72 minutes / 16.1° before sunrise) to nightfall (72 minutes / 16.1° after sunset). See Magen Avraham OC 58:1⁵. Note that this nightfall time is not used for any other purpose.
• Alter Rebbe — amiti sunrise to amiti sunset. See Seder Hachnasat Shabbat, Siddur Torah Ohr².

Dawn (alot hashachar)

The point at which the eastern sky begins to lighten. The site supports several opinions:

• GR"A / Magen Avraham (72 minutes) — sun at −16.1° below the horizon. Traditionally rendered as "72 minutes before sunrise"; the site uses the angle so results scale correctly at all latitudes and times of year.
• Alter Rebbe — sun at −16.9°. This is 72 minutes before hanetz amiti. See Shulchan Aruch HaRav OC 89:1⁶.
• Rav Avraham Chaim Naeh — 120 minutes, sun at −26°; used for certain stringencies such as the beginning of fasts. See Shiurei Tzion⁷. Note that even at moderately high latitudes like Montreal or Northern Europe this zman will not show in the summer, because the sun does not descend below that angle.
• Sefer Bein Hashmashot⁸ — sun at −19.8°, corresponding to the 90-minute figure derived by R' Yechiel Michel Tukachinsky.

Misheyakir (earliest talit and tefillin)

The point at which one can distinguish the blue thread of the tzitzit from the white. This is the earliest time to put on talit and tefillin.

• chabad.org default — sun at −10.2°. Several authorities place this moment at 45 equinoctial minutes before sunrise.
• Sefer Eretz Israel⁹ — sun at −11.5°. See also Kaf haChaim 18:18¹⁰.
• Sefer Nivreshet¹¹ — 52 minutes before sunrise (sun at −11.8°).

Latest Shema (sof zman kri'at shema)

The end of the third halachic hour from the start of the day (per the selected opinion). See Mishna Berachot 1:2¹², per the ruling of R' Yehoshua that we follow; Shulchan Aruch HaRav OC 58:3¹³.

Latest Shacharit (sof zman tefila)

The end of the fourth halachic hour. See Berachot 26a¹⁴ — up to four hours the tefilla is le-chatchila.

Erev Pesach — latest to eat and burn chometz

• Latest time to eat chometz — end of the fourth halachic hour.
• Burn chometz before — end of the fifth halachic hour.

Chatzot — halachic noon and midnight

Chatzot hayom (noon) — the midpoint between sunrise and sunset. Chatzot halayla (midnight) — 12 halachic hours after noon, i.e., the midpoint between sunset and the following sunrise.

Mincha Gedola

6.5 halachic hours from the start of the day. The earliest time one may pray Mincha.

Mincha Ketana

9.5 halachic hours from the start of the day. Preferred earliest time for Mincha le-chatchila.

Plag HaMincha

10.75 halachic hours from the start of the day. The earliest time one may accept Shabbat or daven Ma'ariv (per certain opinions).

Sunset (shkia)

Visible sunset — the sun's upper edge disappearing on the western horizon (−0.833°). This begins bein hashmashot, the disputed period between day and night.

Nightfall (tzait hakochavim)

The end of bein hashmashot — the halachic transition from day to night. Several opinions:

• Three medium stars visible — sun at −6°. This is the default nightfall for weekday purposes and end of fast (except Yom Kippur). See Shulchan Aruch OC 293:2¹⁵.
• Three small stars — sun at −8.5°, 36 equinoctial minutes. See Shulchan Aruch HaRav¹⁶, Igrot Moshe¹⁷, Sefer Bein Hashmashot¹⁸. Used for the end of Shabbat and Yom Tov.
• Rabbeinu Tam — 72 minutes after visible sunset (fixed equinoctial minutes), following Tosafot's calculation of four Roman miles at 18 minutes each. See Tosafot Shabbat 35a "trei tarti gavni". Held stringently for the end of Shabbat by some communities. The site displays it as an optional reference row and suppresses it when it would fall earlier than the primary tzait at extreme latitudes.
• Melamed Lehoil — sun at −7.083°. See Melamed Lehoil¹⁹ OC 30, discussing end of Shabbat in European latitudes.

Candle lighting

• Erev Shabbat and Erev Yom Tov — 18 minutes before sunset by common custom (Jerusalem: 40 minutes). See Shulchan Aruch OC 261:2²⁰ establishes tosefet Shabbat; the specific 18/40 minute intervals are minhag.
• Day 1 of a two-day Yom Tov, when Day 2 is not on Shabbat and begins after sunset, or when Yom Tov occurs after Shabbat — Candles are lit from an existing flame after tzait hakochavim, using three small stars (−8.5°).

End of Shabbat and Yom Tov

Nightfall with three small stars, 36 minutes at −8.5°. This is the same for all major opinions. See Shulchan Aruch OC 293:2¹⁵. When the Rabbeinu Tam row is enabled, it appears alongside as a stringent alternative. Rabbeinu Tam nightfall might not be displayed in high latitudes where it occurs before the time of three small stars.

Magen Avraham at high latitudes

At high enough latitudes (roughly above 50° in the summer/winter extremes) the sun never dips 16.1° below the horizon, so the Magen Avraham dawn does not occur. When this happens, the site sets the halachic hour to two clock hours (equivalent to treating the halachic day as 24 hours long) and reckons all daytime zmanim from halachic midnight. Times computed this way are marked with an asterisk (*) and carry a footnote advising to consult a competent rav before relying on them.

References

1. About Our Zmanim Calculations — chabad.org
2. Seder Hachnasat Shabbat, Siddur Torah Ohr — hebrewbooks.org
3. Biur HaGr"a on Shulchan Aruch OC 459:2 — sefaria.org
4. Sefer Bein Hashmashot 11 on Magen Avraham (p. 98) — hebrewbooks.org
5. Magen Avraham on Shulchan Aruch OC 58:1 — sefaria.org
6. Shulchan Aruch HaRav OC 89:1 — hebrewbooks.org
7. Shiurei Tzion (R' Avraham Chaim Naeh) — hebrewbooks.org
8. Sefer Bein Hashmashot (R' Yechiel Michel Tukachinsky), appendix p. 99 (דף צ״ט) — hebrewbooks.org
9. Sefer Eretz Israel — hebrewbooks.org
10. Kaf haChaim 18:18 — hebrewbooks.org
11. Sefer Nivreshet — hebrewbooks.org
12. Mishna Berachot 1:2 — sefaria.org
13. Shulchan Aruch HaRav OC 58:3 — sefaria.org
14. Berakhot 26a — sefaria.org
15. Shulchan Aruch OC 293:2 — sefaria.org
16. Shulchan Aruch HaRav OC 293:2 — sefaria.org
17. Igrot Moshe OC 4:62 — hebrewbooks.org. R' Moshe Feinstein writes that the longest bein hashmashot in New York is 50 minutes, corresponding to 43 minutes in Jerusalem (per Sefer Bein Hashmashot), which in turn is equivalent to 36 minutes in Jerusalem at the equinox — the sun depression that yields the −8.5° angle used here.
18. Sefer Bein Hashmashot on 3 small stars — hebrewbooks.org
19. Melamed Lehoil OC 30 (R' Dovid Zvi Hoffmann) — hebrewbooks.org
20. Shulchan Aruch OC 261:2 — sefaria.org

Date calculations

Birthday

A Hebrew birthday is generally observed on the same Hebrew date each year. Two exceptions:

• Born in Adar of a non-leap year — in a leap year the birthday is observed in Adar II.
• Born on 30 Cheshvan or 30 Kislev — in years where that month has only 29 days, the birthday rolls forward to 1 Kislev (respectively 1 Tevet), which is Rosh Chodesh.

Yahrzeit

The yahrzeit is generally observed on the same Hebrew date each year. The exceptions differ from the birthday rule:

• Passed away in Adar of a non-leap year — most authorities today observe the yahrzeit in Adar I of a leap year, following the Rama; this is what we compute. The Shulchan Aruch and some Sephardic authorities rule Adar II. In the first year after passing, however, Adar I is preferred by many of those opinions who otherwise rule in favor of Adar II, since exactly 12 months must have elapsed (Magen Avraham there). See Shulchan Aruch OC 568:7¹.
• Passed away in Adar I of a leap year — subsequent leap years: Adar I; non-leap years: the sole Adar.
• Passed away in Adar II of a leap year — subsequent leap years: Adar II; non-leap years: the sole Adar.
• Passed away on 30 Cheshvan or 30 Kislev — the rule depends on what happened in the first year after death:
  • If that year had a 30th of the same month (a full month), then in future years with only 29 days the yahrzeit is observed on Rosh Chodesh (1 Kislev or 1 Tevet respectively) — same as the birthday rule.
  • If that year did not have a 30th (the month was deficient), the yahrzeit is observed on the last day of that month every year — 29 in deficient years, 30 when the month is full.
  See also Kitzur Shulchan Aruch 221².

Other recurring events

Bar / Bat mitzvah, wedding anniversaries and other recurring Hebrew-date events follow the birthday rule above.

The 19-year cycle

The Hebrew calendar reconciles the lunar month with the solar year by inserting a leap month (Adar I) in 7 of every 19 years. Over the full cycle, 19 solar years equal almost exactly 235 lunar months (12 × 12 + 7 × 13), so months and seasons stay aligned.

Within each 19-year cycle, the leap years are years 3, 6, 8, 11, 14, 17 and 19. This is remembered by the mnemonic גו״ח אדז״ט, where each Hebrew letter is its numeric position modulo 10.

The current Hebrew year's position in the cycle is:

((year − 1) mod 19) + 1

See Rambam, Hilchot Kiddush HaChodesh 6:11³.

Computing the day of Rosh Hashana

The molad (new moon) of Rosh Hashana is computed from the traditional constants: the mean synodic month is 29 days 12 hours 793 chalakim (1 chelek = 1/1080 hour ≈ 3⅓ seconds). Molad Tishrei of any year is projected from Molad Beharad — the reference molad of Nisan year 1, which fell on Monday at 5 hours 204 chalakim after 6 PM Sunday. Note that this molad (and almost the entire year 1) is before creation according to the tradition that the world was created in Tishrei (Rabbi Eliezer in Rosh Hashana 10b⁴), and is sometimes called molad tohu.

Once the molad is known, Rosh Hashana can be postponed by up to two days via the four dechiyot (postponement rules):

1. Molad zaken (מולד זקן) — if the molad falls at noon or later, Rosh Hashana is pushed to the next day.
2. Lo ADU rosh (לא אד״ו ראש) — Rosh Hashana never falls on Sunday, Wednesday or Friday. If the molad (or the day after molad zaken) lands on one of these, push forward one day.
3. Gatarad (גטר״ד — ג=Tue, ט=9h, ר״ד=204ch) — in a non-leap year, if the molad falls on Tuesday at 9 hours 204 chalakim or later, Rosh Hashana is pushed to Thursday (via lo ADU rosh).
4. Betutakpat (בט״ו תקפ״ט — ב=Mon, ט״ו=15h, תקפ״ט=589ch) — in the year immediately after a leap year, if the molad falls on Monday at 15 hours 589 chalakim or later, Rosh Hashana is pushed to Tuesday.

See Mishneh Torah — Hilchot Kiddush HaChodesh, chapters 6–8³.

Parshiyot

The schedule of weekly Torah readings is governed by the rules set out in Tur, Orach Chaim 428⁵. There are 14 year types, determined by: (1) the day of the week of Rosh Hashana, (2) whether Cheshvan and Kislev have their standard number of days, and (3) the day of the week of the first day of Pesach. The rules for merging and splitting certain parsha pairs were codified by R. Chaim Mordechai haCohen Brecher and are printed in the back of some Chumash editions (see table⁶).

Each year type is encoded with three Hebrew letters: the day of Rosh Hashana (ב Mon · ג Tue · ה Thu · ז Sat), the year length (ח deficient · כ regular · ש complete), and the day of the first day of Pesach. This code appears in Chumashim and Tikun Korim.

In some years certain readings in Eretz Israel differ from the Diaspora because a second day of Yom Tov displaces a reading in the Diaspora. The Parshiot page shows both readings.

Tekufot and Birkat HaChama

Tekufot are the four astronomical seasons: Tekufat Nisan (spring equinox), Tekufat Tammuz (summer solstice), Tekufat Tishrei (autumnal equinox) and Tekufat Tevet (winter solstice). Two calculations are traditional:

• Tekufat Shmuel — a simple year of exactly 365¼ days, giving 91⅜ days per season. This is what governs practical halachic reckoning today (Tal uMatar in the Diaspora, Birkat HaChama). See Eruvin 56a⁷; Rambam, Kiddush HaChodesh 9:4³.
• Tekufat Rav Ada — matches the mean solar year underlying the molad calculation (365 days, 5h, 55m, 25 25⁄57 sec); more accurate but retained mostly for theoretical reference. See Rambam, Kiddush HaChodesh 10:1³.

Birkat HaChama (blessing on the sun) is recited every 28 years, on the morning after Tekufat Nisan (per Shmuel) falls on Tuesday evening / Wednesday sunrise. The last occurrence was Wednesday 8 April 2009; the next is 8 April 2037.

This is because, according to Shmuel, Tekufat Nisan advances by 1¼ days in the weekly cycle every year, so once in 28 years it falls on Tuesday at 6 AM — the hour the sun was traditionally created. There is no associated astronomical event; it is the halachically accepted tradition.

See Mishneh Torah — Hilchot Kiddush HaChodesh, chapter 10³.

References

1. Shulchan Aruch OC 568:7 — yahrzeit for one who died in Adar (with Rama gloss, Magen Avraham and Kaf HaChaim) — sefaria.org
2. Kitzur Shulchan Aruch 221 on Yahrzeit — sefaria.org
3. Mishneh Torah — Hilchot Kiddush HaChodesh — sefaria.org
4. Rosh Hashana 10b — Rabbi Eliezer: the world was created in Tishrei — sefaria.org
5. Tur, Orach Chaim 428 — R. Yaakov ben Asher, summarizing the rulings of his father R. Asher ben Yechiel (the Rosh) — sefaria.org  |  traditional print edition — hebrewbooks.org
6. R. Chaim Mordechai haCohen Brecher — Parsha pairing/splitting table — hebrewbooks.org
7. Eruvin 56a — Shmuel's astronomical year of 365¼ days — sefaria.org