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This page explains what a zman is, why the halachic hour stretches and shrinks with the seasons, why poskim land on different times for the same moment, and how the day begins. It is definitional only. It explains what these times are and where they come from, never what you should do with them. For any practical question, ask your rav. The times shown elsewhere on this site are computed by software. Build in a margin rather than relying on them to the last minute.
זמן (zman) literally means "time." In this context it means one specific, halachically significant moment inside the day: the earliest moment at which the morning Shema may be recited, the moment the sun is exactly overhead, the moment three stars become visible. "Zmanim" is the plural: the whole set of these moments, strung through a single day from before dawn to after dark.
None of these moments is arbitrary. Each one traces back to a Talmudic source, a verse, or an observed natural event (first light, sunrise, midday, sunset, the emergence of stars) and later poskim translated that description into a measurable calculation. PowerZmanim's job is to compute those calculations correctly and show you where each one comes from, not to decide which one is right for you.
Most zmanim are not counted in fixed clock hours. They're counted in sha'ot zmaniyot: proportional, or "halachic," hours. Each day's daylight (or, for some zmanim, the night) is divided into twelve equal parts, and a zman is defined as a certain number of those parts from the start of the day, rather than as a fixed clock time.
Because the length of daylight itself changes with the seasons, one sha'ah zmanit is not a fixed 60 minutes. It's a twelfth of however long that day's daylight actually is: longer in the summer, shorter in the winter. Sof zman keriat Shema, for example, is always "three sha'ot zmaniyot into the day", but three sha'ot zmaniyot land at a different clock time in June than they do in December, because each sha'ah zmanit is a different number of minutes long.
A sha'ah zmanit can be measured across different definitions of "the day," which is itself a source of machloket: the Vilna Gaon counts from sunrise to sunset, the Magen Avraham counts from alot hashachar to tzeit hakochavim. The next section walks through why.
PowerZmanim shows several numbers for what feels like "the same" zman: two versions of sof zman Shema, five versions of tzeit hakochavim. That isn't the app hedging; it's an honest reflection of real, longstanding disagreement among poskim about how to translate a Talmudic description into a measurement. Three examples below, sourced from the same registry that drives every number on this site.
The halachic day does not run midnight to midnight, the way the civil calendar does. It begins and ends at nightfall, roughly at tzeit hakochavim. That's why a Hebrew date like the 15th of the month starts the evening before the corresponding civil date would suggest, and why Shabbat begins Friday evening rather than Saturday morning.
This is also why the prefix erev ("evening") is used for the day leading into Shabbat or a holiday (erev Shabbat, erev Pesach), since the halachic day has already begun once the sun sets the evening before. PowerZmanim's Hebrew-date display follows the same rule: it advances at tzeit hakochavim, not at midnight, and says so when it does.
Because "nightfall" itself is one of the zmanim that poskim disagree about (see tzeit hakochavim in the glossary below, and the five tzeit opinions on the methodology page), exactly when a given halachic date starts can differ by close to an hour depending on which shita you follow.
The home page draws the halachic day instead of just listing it. Two views, plus one choice that is yours to make — all driven by the same computed times as the text table underneath them:
A short, prioritized list of what's closing next: not all twenty-two zmanim at once, just the next few transitions and how much time remains in each. It's driven by the same live clock as the day explorer, so scrubbing the day updates this list too.
The full local civil day drawn as a sky, midnight to midnight, with the sun on its real path and each milestone marked at its true time. Dragging along it previews any moment of the day; the marker returns to the present when you go back to live. Because the Hebrew date turns at shkia rather than at midnight, that change is drawn separately as its own band, split at the calculated shkia, instead of bending the civil axis to hide the difference.
Nightfall is not one time. Five tzeit hakochavim opinions are listed side by side in the full table, and none of them is bold, preselected or recommended until you choose which shita you follow. That choice lives in Settings, under "My tzeit line"; once it is made, your line is the one marked throughout the rest of the site, and clearing it puts the whole page back to neutral. Which shita applies to you is a question for your rav.
Every colored view on this site ships with a legend and a full text-table equivalent underneath it: the table is never optional decoration, it's the same data, spelled out.
Every Hebrew term used on this site, defined in one place.