How Time Is Measured

Zmanim methods do not all measure time in the same way. Understanding the measurement type explains why two calendars can display different results.

Sha’ah regilah: regular clock time

A sha’ah regilah is an ordinary 60-minute clock hour. A dakah regilah is an ordinary clock minute of 60 seconds. A rule such as “twenty regular minutes after sunset” therefore remains twenty clock minutes throughout the year.

Sha’ah zmanit: proportional time

A sha’ah zmanit is one twelfth of a specifically defined halachic day. A dakah zmanit is one sixtieth of that proportional hour, or one 720th of the defined day. These are also sometimes called a seasonal hour and seasonal minute. iKaluach uses “proportional” because it states the mathematical relationship more clearly.

The defined day might run from Hanetz hachama to Shkiat hachama or from Alot to a particular Tzeit. The choice of endpoints is part of the shita. Two methods can therefore produce different proportional hours on the same date and at the same location.

Sha’ah zmanit l’chumra: strict proportional time

A strict proportional hour uses whichever is longer: the calculated proportional hour or a regular 60-minute hour. It may therefore be longer than a regular hour, but never shorter. A strict proportional minute follows the same rule: it is the longer of the calculated proportional minute and one regular clock minute.

This convention is useful when the underlying rule might require regular or proportional time and the later result is desired for stringency—for example, in methods that apply it to Mincha Gedolah.

Equivalent minutes and solar angles

In iKaluach, equivalent minutes are another way of naming an angle-based calculation. A traditional minute interval is treated as the interval observed under a reference condition—commonly Eretz Yisrael near an equinox—and translated into the sun’s position below the horizon.

The app then finds that same solar angle for the actual date and location. The displayed clock interval can therefore be longer or shorter than the named number of equivalent minutes. “72 equivalent minutes,” for example, does not mean 72 fixed clock minutes and is not a proportional-minute calculation.