Why Zmanim Differ Between Calendars

A difference of a minute or two does not necessarily mean that one calendar has misplaced the sun.

Zmanim combine astronomical calculation with halachic definitions. Two careful calendars can therefore display slightly different results.

Common reasons for a difference

• Different halachic methods.
• Slightly different latitude, longitude, or elevation values.
• Different treatment of the visible horizon, sea level, and atmospheric refraction.
• Different astronomical algorithms or reference data.
• Different rounding and display rules.
• A device clock or time-zone database that is not current.

Precision is not the same as certainty

An app can display seconds, but that does not make every underlying halachic and astronomical assumption exact to the second. The appropriate margin depends on the mitzvah and on whether the zman is an earliest permitted time or a final deadline.

When a few minutes matter, identify the selected method and follow practical rabbinic guidance rather than applying one universal adjustment to every zman.

Historical tables

Some established Israeli calendars preserve historical observation tables or locally developed practices. They should be used according to the places and conventions for which they were prepared. Kaluach’s calculated values may therefore differ from a printed local table even when both are internally consistent.