GUIDE
Why zmanim times differ between calendars
Put two luachs or apps side by side and the zmanim often don’t match. Usually neither is wrong: they are using different locations or different opinions. Here are the most common reasons, so you can tell a settings difference from a real mistake.
1. Location
Sunrise and sunset depend on your exact position. Moving east or west changes them by about a minute for every 12 to 13 miles (about 20 km) at the latitude of New York, and moving north or south changes them by a different amount depending on the season. A calendar set to the city center can differ slightly from one set to the shul’s address. Time zone and daylight saving time also have to be set correctly.
2. Elevation
From higher ground you see the sun rise earlier and set later. Some calendars use sea-level sunrise and sunset, and others adjust for elevation. In hilly areas this alone can move times by a minute or more.
3. Candle lighting customs
Candle lighting is a fixed number of minutes before sunset, and the number depends on the community. 18 minutes is the most common custom. In Jerusalem the custom is 40 minutes, and some other communities use 20, 22 or 30 minutes. A flyer that uses a different number from the app you compare it with will be several minutes apart, and both are correct for their own custom.
4. When Shabbos ends (tzeis hakochavim)
There are several opinions for nightfall. Common ones are:
- Degrees below the horizon, for example when the sun is 8.5° below the horizon. The number of minutes after sunset this works out to changes with the season and your latitude.
- A fixed number of minutes after sunset, such as 42, 50 or 72 minutes.
- Rabbeinu Tam, traditionally 72 minutes after sunset, which many flyers list as a separate, later time.
Two calendars using different opinions can be many minutes apart, especially in summer.
5. Sof zman krias shema and other daytime zmanim
Daytime zmanim are measured in “halachic hours”: the day divided into twelve. The Gra measures the day from sunrise to sunset. The Magen Avraham measures it from dawn (alos hashachar) to nightfall, so his times are earlier. Dawn itself also has more than one definition, for example 72 minutes before sunrise or the sun at 16.1° below the horizon. Each choice moves the result.
6. Rounding
Calculated times include seconds. Many calendars round to be on the safe side: candle lighting down to the earlier minute, and the end of Shabbos up to the later minute. Others round to the nearest minute. That alone explains many one-minute differences.
What to do about it
- Decide the settings with your rav: location, candle-lighting minutes and the opinions for tzeis and krias shema.
- Write them down and use the same source and settings every week.
- Label the times on the flyer clearly, for example “Shabbos ends” and “Rabbeinu Tam”, or “Sof zman krias shema (Magen Avraham)”.
- Investigate any difference over a minute against a second source before publishing.
WeeklyZmanim sets each flyer up with its organization’s approved location and calculation settings, uses Hebcal as the primary reference, and holds a flyer for review if equivalent times are more than one minute apart. A check like this is not a guarantee of halachic correctness: your rav’s decisions on the settings are what matter.
This guide explains common calculation differences. It is not a halachic ruling. Ask your rav which opinions your community follows.