Zmanim settings

More accurate zmanim, explained

Both zmanim settings follow the Alter Rebbe, the way Chabad.org's calendar does. More accurate is now Kosher.Chat's default, and it goes after the last few seconds: it works out the Alter Rebbe's own words the way they happen in real life, and uses finer math. Chabad.org's times are right to the minute for everyday use, and you can switch to them any time. Here is every difference, with the numbers.

What changes on your screen

More accurate compared with Chabad.org, in Crown Heights over a full year.

Dawn (alos)95 to 163 s later
MisheyakirSame, within 5 s
SunriseSame, within 4 s
Latest Shema29 to 38 s later
Latest Shacharis18 to 26 s later
Midday (chatzos)Same, within 4 s
Mincha gedola2 to 7 s earlier
Mincha ketana33 to 43 s earlier
Plag hamincha46 to 59 s earlier
Candle lighting21 to 29 s earlier
Sunset21 to 29 s earlier
Nightfall (tzeis)113 to 146 s earlier
Shabbos ends (havdalah)Same moment
MidnightSame, within 4 s

Shown to the minute, most of these come out the same or one minute apart. Dawn and nightfall are usually two minutes apart. Farther north the seconds grow a little; nearer the equator they shrink. One honest caveat: the air bends sunlight a little differently every evening, so on any given day sunrise and sunset can shift by about half a minute either way (see “Today's weather” below). Treat every time, in either setting, as good to about half a minute.

The Alter Rebbe's words, the way they happen in real life

The Alter Rebbe defines true sunrise and sunset by sunlight on the tops of the high mountains of Eretz Yisrael, and the sunset we see by sunlight on the treetops. These are about measuring those moments the way they really happen.

Light bends on its way up the mountain
How far below the horizon the sun can be and still light the mountaintop.
Chabad.org uses
0.75°, a straight line
More accurate uses
0.82°, light that bends like real light
24 to 28 s true sunrise earlier, true sunset later
Light bends on its way up the mountain
In short
Light doesn't travel through the air in a perfectly straight line. It curves gently along the Earth, so a mountaintop keeps catching the sun a little longer than a straight-line drawing says.
A straight line from the peak to the setting sun would hit the curve of the Earth. Real light bends over it, so the peak still glows.
The details
The Alter Rebbe's true sunset is when the light leaves “the tops of the high mountains of Eretz Yisrael”. In 1975 Rabbi Sholom DovBer Levine worked this out with Mount Carmel, 546 m (1,790 ft) high, the mountain the Gemara uses for sunset.
He drew the last ray from the sun to the peak as a straight line. That puts the sun 0.75° below the sea-level horizon when the ray leaves the peak. Real light bends down a little the whole way, so the peak keeps it until the sun is 0.82° down. It's the same bending that lets you see the sun for about two minutes after it has really set.
More accurate uses the standard formula for a ray grazing the Earth (Sweer, 1938, as built into the astronomy-engine library).
True sunrise, on its own
24 to 28 s earlier
True sunset, on its own
24 to 28 s later
יניח חמה בראש הכרמל
Let him place the sun at the top of the Carmel.
Gemara, Shabbos 35a
חמה סמוך לשקיעתה נראית על ראשי ההרים
Close to its setting, the sun is seen on the mountaintops.
Rashi there
The 546 m calculation: a straight tangent from the summit, 45′, plus 50′ for sunset at sea level.
Rabbi S.D. Levine, Kuntres Neros Shabbos Kodesh, part 2 (1975), p. 9
When does the light leave the mountaintop?
The Alter Rebbe's words are “when the light leaves”. How much of the sun has set by then?
Chabad.org uses
The whole sun gone
More accurate uses
Half the sun gone, when the light really fades
84 to 99 s true sunrise later, true sunset earlier
When does the light leave the mountaintop?
In short
Chabad.org's angle waits for the very last sliver of the sun to set from the peak. But in real life the light on a peak already looks like shade once about half the sun is down. Rabbi Levine saw this himself.
Seen from below, the peak stops glowing well before the last sliver sets.
אזי היא שקיעה האמיתית, שהוא סילוק וביאת האור מראשי ההרים הגבוהים שבארץ ישראל
That is the true sunset, which is when the light leaves the tops of the high mountains of Eretz Yisrael.
The Alter Rebbe, Seder Hachnasas Shabbos
What Rabbi Levine found
He went and looked. Once most of the sun has set, he writes, there is almost no visible difference between sunlight and shade on the mountain, so true sunset must come at least a minute earlier than his tables. In his 2013 notes he adds that at that moment, someone standing on the peak can still see close to half the sun.
שוב אין נראה כמעט שום חילוק בין אור השמש לצילו
There is hardly any visible difference anymore between the sunlight and its shade.
Rabbi S.D. Levine, Dover Shalom (2003), p. 152 (קנב)
יוכל לראות עדיין קרוב לחצי גלגל השמש
He can still see close to half the sun's disk.
Rabbi S.D. Levine, notes to Seder Hachnasas Shabbos (Kehot 2013), note 10, p. 10 (י)
He left his printed tables as they were, calling it too small a change to pin to a time. More accurate takes that moment: the sun's center on the mountaintop's horizon, half above it and half below. It fits the physics too: at the horizon, sunlight crosses almost 40 times as much air as at noon, so half a sun barely stands out from the glowing sky.
Together with the bending
True sunrise and sunset come when the sun is 1.39° below the horizon instead of 1.58°. The halachic day gets 2 to 2½ minutes shorter: true sunrise 61 to 73 s later, true sunset 61 to 73 s earlier. Every zman counted in halachic hours moves with it:
Latest Shema
30 to 36 s later
Latest Shacharis
20 to 24 s later
Mincha ketana
35 to 42 s earlier
Plag hamincha
48 to 57 s earlier
The sunset you see
The time printed as “sunset”, and candle lighting 18 minutes before it.
Chabad.org uses
0.83°: the last sliver at a sea-level horizon
More accurate uses
0.76°: light leaving the treetops, the Alter Rebbe's sunset
23 to 28 s sunset and candle lighting earlier
The sunset you see
In short
The Alter Rebbe describes the sunset we see as the moment sunlight leaves the treetops and tall roofs in open, flat land. Measured the same way as the mountaintop, that comes about 25 seconds before the last sliver of the sun drops below a sea-level horizon, which is the “sunset” most calendars print.
Candle lighting is 18 minutes before sunset in both settings, so it moves earlier by the same amount.
להדליק קודם שקיעת החמה, שקיעה הנראית, דהיינו בעוד השמש זורח בראשי האילנות בשדה בארץ המישור שאין שום הר במערב, או בראשי גגים הגבוהים בעיר
Light before sunset, the visible sunset: while the sun still shines on the treetops in a field, in flat land with no mountain to the west, or on the tops of tall roofs in town.
The Alter Rebbe, Seder Hachnasas Shabbos
The details
Rabbi Levine takes the treetops and tall roofs as 30 m (about 100 ft). From up there, bent light lingers until the sun is about 0.19° lower than at sea level. But just as on the mountain, the light looks gone once half the sun is down, which is 0.27° sooner. So the treetops go dark a little before the last sliver leaves a sea-level horizon.
Sunrise stays at the usual sea-level time. The Alter Rebbe gives no treetop rule for sunrise, and mirroring this one would make sunrise earlier, the lenient side.
Sunset
23 to 28 s earlier
Candle lighting
23 to 28 s earlier
Dawn (alos hashachar)
How far below the horizon the sun is at dawn.
Chabad.org uses
16.9°
More accurate uses
16.6°
95 to 161 s dawn later (longer in summer)
Dawn (alos hashachar)
In short
The Alter Rebbe's dawn is 72 minutes before true sunrise, measured in Jerusalem on the day of the equinox. To use it anywhere and in any season, it becomes an angle: how far below the horizon the sun is at that moment. True sunrise moved, so the dawn angle moves too: 16.6° instead of 16.9°.
72 minutes is 4 mil of 18 minutes each. On the equinox in Jerusalem, 72 minutes before true sunrise the sun is 16.6° down. That angle is then used everywhere, all year.
מסתבר יותר, שחושבים את 72 הדקות מעת נץ החמה האמיתי
It makes more sense to count the 72 minutes from true sunrise.
Rabbi S.D. Levine, notes to Seder Hachnasas Shabbos (Kehot 2013), note 92, p. 51 (נא)
Where Chabad.org's 16.9° comes from
It matches 76 minutes before the sunrise we see, in Jerusalem on the equinox, as one of the letters printed in the same book counts it (p. 75, עה). Counted the Alter Rebbe's way, 72 minutes from Chabad.org's own true sunrise, it would be 16.79°.
Dawn
95 to 161 s later
Most in
early summer, when the sun climbs at a slant
Nightfall (tzeis hakochavim)
How far below the horizon the sun is when three medium stars come out.
Chabad.org uses
6°
More accurate uses
5.64°
114 to 145 s nightfall earlier; Shabbos still ends at the same time
Nightfall (tzeis hakochavim)
In short
Nightfall is 20 minutes after true sunset, measured in Jerusalem on the equinox. True sunset moved about a minute earlier, so nightfall's angle moves too: 5.64° instead of 6°. Shabbos and yom tov still end at the havdalah time, which doesn't change.
On the equinox in Jerusalem, 20 minutes after true sunset the sun is 5.64° down.
וזמן צאת הכוכבים הבינונים הוא כשהשמש יורדת 5°50' תחת האופק (20 דקות אחרי השקיעה האמיתית)
Medium stars come out when the sun is 5°50′ below the horizon (20 minutes after true sunset).
Rabbi S.D. Levine, notes to Seder Hachnasas Shabbos (Kehot 2013), p. 21 (כא)
That is Rabbi Levine's own method, with his true sunset. Chabad.org's 6° rounds a different figure, 5.95° (24 minutes after the sunset we see), which Rabbi Levine sets aside on the same page. With the true sunset above, his method gives 5.64°.
Nightfall
114 to 145 s earlier
Shabbos and yom tov end
unchanged (havdalah time, 8.5°)

Finer math

These don't change any definition. They just compute the same thing more exactly.

Where the sun is
Every zman starts from knowing exactly where the sun is in the sky.
Chabad.org uses
NOAA's formula, up to 3.5 s off NASA
More accurate uses
A finer model, within 0.25 s of NASA
about 1.1 s, up to 3.5 s either way
Where the sun is
In short
Chabad.org uses NOAA's sun formula, accurate to about a second. More accurate uses the astronomy-engine model, checked against NASA's own record of the sun to within a quarter second.
Seconds off NASA JPL's DE421 ephemeris, over a full year in Crown Heights.
What's left is mostly the Earth's own tiny wobbles in its spin, which nobody can predict years ahead. More accurate also uses the sun's real size each day: it looks about 3% bigger in January, when the Earth is closest to it.
Rounding to the minute
Calendars show whole minutes, so every time gets rounded.
Chabad.org uses
The nearest minute, for sunrise, sunset, candle lighting and havdalah
More accurate uses
Always the safe way: never early for a start, never late for a deadline
up to 30 s on the minute shown
Rounding to the minute
In short
Rounding to the nearest minute can show a time up to 30 seconds on the lenient side. More accurate always rounds the safe way.
A sunrise at 7:21:29. The nearest minute is 7:21, half a minute before the sun is really up.
Start times (sunrise, mincha, havdalah) round up. Deadlines (Shema, sunset, candle lighting) round down. Chabad.org already does this for most zmanim; sunrise, sunset, candle lighting and havdalah are the exceptions.
Dawn in the far north in summer
Places like London, where the summer sun never sinks low enough for the dawn angle.
Chabad.org uses
12 hours before midday
More accurate uses
The sun's real lowest point of the night
up to 25 s only far north, around summer
Dawn in the far north in summer
In short
When the sun never gets low enough for dawn, Chabad.org's own article says dawn begins when the sun passes its lowest point of the night. More accurate finds that exact moment. Chabad.org's site uses 12 hours before midday, which can be up to 25 seconds off.
London in late June: the sun bottoms out about 15° down, never the 16.6° that dawn needs.
Midday (chatzos)
The middle of the day, which mincha gedola counts from.
Chabad.org uses
The middle of sunrise and sunset
More accurate uses
The middle of true sunrise and true sunset
under 1 s either way
Midday (chatzos)
In short
Chabad.org's article puts midday halfway between true sunrise and true sunset; its site uses ordinary sunrise and sunset. Both stretch the day by the same amount on each side, so the middle lands within a second.
Wider or narrower, the two days share the same middle.
Mincha gedola follows the article too: 6½ halachic hours after true sunrise, rather than half an hour after midday.

Looked at, but not used

Good questions we checked, and why the answer stays the same.

Taller mountains
Which “high mountains of Eretz Yisrael” does the Alter Rebbe mean?
We use
Mount Carmel, 546 m, as Rabbi Levine does
The other view
The hills around Jerusalem, 700 to 800 m (Rabbi Feigelstock)
92 to 110 s true sunset later than ours, if used
Taller mountains
In short
Rabbi Yosef Yitzchok Feigelstock reads “the high mountains” as the taller hills around Jerusalem, and the Alter Rebbe's “about four minutes” as measured in Eretz Yisrael. His letter in the 2013 edition puts true sunset later. It is a real disagreement.
Higher mountains keep the light longer, so true sunset would come later.
Why we stay with Rabbi Levine
The Alter Rebbe writes that where he lived, stars come out half an hour after true sunset, which is 34 minutes after the sunset we see: a four-minute gap in “these countries”. Rabbi Levine's Carmel gives about four minutes there. It is also the reading Chabad.org and most Chabad calendars follow.
ובמדינות אלו זמן צאת ג' כוכבים בינונים בימים השוים הוא בכמו חצי שעה בקירוב אחר שקיעה האמיתית, שהן ל"ד חלקי ששיים בקירוב אחר שקיעה הנראית
In these countries, three medium stars come out at the equinox about half an hour after true sunset, which is about 34 minutes after the visible sunset.
The Alter Rebbe, Seder Hachnasas Shabbos
Today's weather
How much the air bends light changes with temperature and pressure.
We use
The standard average, as every calendar does
The other view
Each day's actual air
about 35 s either way on a typical day; sometimes 2 min or more
Today's weather
In short
A cold, clear evening can keep the sun in view a bit longer, a hot hazy one a bit less. Nobody can know tomorrow's air today, so every calendar, and both of our settings, use the standard average.
When 135 sunsets were timed in Edmonton, the bending varied by about 0.11° from day to day. In Crown Heights that is about 35 seconds either way on a typical day, and on unusual evenings two minutes or more. That's one more reason to leave yourself a few minutes.
Refraction at the horizon averaged 0.58°, with a spread of 0.11°.
Peterson, Lozowski, Hube and Sampson, Publications of the Astronomical Society of the Pacific (2003)
Hills, buildings and your own height
A hill to the west hides the sun sooner; a high floor sees it longer.
We use
Sea level, open flat land
The other view
Your spot's real height and skyline
about 90 s per 100 m (330 ft) of height, if it counted
Hills, buildings and your own height
In short
The Alter Rebbe already tells us how to measure: sunset in an open field in flat land with no mountain to the west, and true sunset by the mountains of Eretz Yisrael wherever you are. So a hill nearby, or living on a high floor, doesn't change these zmanim. Like every printed calendar, both settings give times for sea level.

Which setting should I use?

More accurate is the default, for the Alter Rebbe's definitions worked out to the second. Some of its times come a little earlier than Chabad.org's and some a little later. If your shul or community follows Chabad.org's calendar, switch to the Chabad.org setting, which matches it to the minute. For questions of halacha in practice, ask your rav. Either way, don't cut it close: leave yourself a few minutes.

You can switch any time under the gear icon at the bottom of the chat sidebar, under Zmanim.

Sources

The Alter Rebbe, Seder Hachnasas Shabbos, printed in his Siddur. Kehot edition (2013) with notes and letters edited by Rabbi Sholom DovBer Levine.
Rabbi S.D. Levine, Kuntres Neros Shabbos Kodesh, parts 1 and 2 (1975), and Dover Shalom (2003), pp. 150 to 152.
Rabbi Y.Y. Feigelstock, letter in Seder Hachnasas Shabbos (Kehot 2013), pp. 93 to 109.
Gemara Shabbos 35a, with Rashi.
Chabad.org, About Our Zmanim Calculations.
NASA JPL's DE421 ephemeris, and the astronomy-engine library, which uses Sweer's (1938) formula for bent light.
Peterson et al. (2003) and Sampson et al. (2005), on how much sunset refraction varies.

All seconds are for Crown Heights from November 2026 to October 2027, worked out from each setting's own code. Updated October 2026.

What changes on your screen

The Alter Rebbe's words, the way they happen in real life

Light bends on its way up the mountain

When does the light leave the mountaintop?

The sunset you see

Dawn (alos hashachar)

Nightfall (tzeis hakochavim)

Finer math

Where the sun is

Rounding to the minute

Dawn in the far north in summer

Midday (chatzos)

Looked at, but not used

Taller mountains

Today's weather

Hills, buildings and your own height

Which setting should I use?

Sources