Hebrew calendar
Enable an AI agent to identify a Hebrew calendar regime, assess whether a date or calendar calculation is valid and sufficiently specified, and determine which conversions or scheduling actions are justified.
Research draft, second pass
A second pass drafted this model: the structure a model of this thing needs, and what is known about it in the world. The line under this one says how the second half was obtained - researched against sources, or recalled without web access, in which case nothing here was read anywhere and every claim is a lead to verify. Unreviewed either way.
Researched by: Codex + Grok
Purpose and description
Enable an AI agent to identify a Hebrew calendar regime, assess whether a date or calendar calculation is valid and sufficiently specified, and determine which conversions or scheduling actions are justified.
The Hebrew calendar is the lunisolar calendar of rabbinic Judaism: months follow a conventional mean lunar conjunction (the molad), years are kept near the solar seasons by inserting a second Adar in seven years of a nineteen-year cycle, and 1 Tishrei is further constrained by weekday postponement rules (deḥiyyot), with years numbered from a conventional era of creation (Anno Mundi).
It can be Validate a Hebrew date against its identified calendar regime and year structure.; Normalize Hebrew-script and transliterated date expressions while retaining unresolved ambiguity.; Convert dates or instants with explicit calendar, location, and day-boundary assumptions.; Compute month sequences and calendar-derived dates using an identified rule set.; Schedule Hebrew-date recurrences using an explicit Adar and missing-day policy.; Compare competing date assignments and explain which assumptions cause the difference..
Distinguishing features
Test whether the rules coordinate lunar months with the solar year through intercalation; a purely lunar calendar without this adjustment is a different system.
For the standard fixed Hebrew calendar, test whether leap years follow the specified seven positions in a 19-year cycle and introduce an additional Adar month.
Test whether the system distinguishes the Tishrei year-number transition from month numbering that begins with Nisan.
Test whether date assignment accounts for the Hebrew day's evening boundary rather than assuming a midnight rollover.
Test whether fixed-calendar calculations account for postponement rules and variable Cheshvan and Kislev lengths; alternating month lengths alone do not identify a valid Hebrew year.
Scope
+ Identification of the calendar tradition, governing rules, and historical applicability
+ Year numbering, leap-year status, month sequence, and variable year lengths
+ Hebrew date expressions, month-name aliases, and date validity
+ Day boundaries and the location and time assumptions required to assign a date
+ Conversion to other calendars and scheduling of Hebrew-date recurrences
+ Calendar-derived observance dates and dependencies on community conventions
- Religious obligations, permissions, and authoritative rulings about observance
- Liturgical texts and synagogue service procedures
- General astronomy, solar-position calculation, and lunar ephemerides
- Civil time-zone administration and the internal rules of other calendars
- Personal life events and historical events dated using the calendar
Characteristics
- Calendar regime
- Standard fixed rabbinic calendar; historically attested observational regime; other explicitly identified tradition; unresolved Determines which rules may be used and whether a computed date represents historical practice or a retrospective calculation.
- Rule authority and applicability
- Relationship to an identified rule source, community or authority, and period of applicability Allows an agent to justify a calculation and detect conflicting conventions.
- Hebrew year and era
- Integer year with an explicit era and policy for omitted thousands Prevents ambiguous year expressions from silently resolving to the wrong date.
- Metonic cycle position
- Position 1-19 under an explicitly stated fixed-calendar indexing convention Supports checking whether the year requires an intercalary month.
- Year structure
- Common or leap; deficient, regular, or complete; unresolved Determines the available months and their lengths under the standard fixed calendar.
- Month identity
- Named Hebrew month, distinguishing Adar in a common year from Adar I and Adar II in a leap year Prevents month-number conventions and Adar ambiguity from corrupting date matching or recurrence.
- Day-boundary context
- Relationship to location, time zone, solar-boundary method, and treatment of twilight Determines whether an instant can be assigned a Hebrew date with sufficient confidence.
- Date resolution state
- Valid and resolved; invalid; incomplete; convention-dependent; boundary-ambiguous Controls whether the agent may convert or schedule directly, must return alternatives, or must request clarification.
Also called
Where this came from
wikidata · CC0 1.0
Drafted structure
Bundle to layer to finding to question, as the second pass will find it: 5 bundles · 9 layers · 16 findings · 24 questions.
Calendar Regime and Authority Identify which Hebrew calendar rules a representation or calculation claims to follow.
The name Hebrew calendar alone does not establish historical applicability or authorize using one tradition's rules for another.
Fixed and Historical Reckoning
Separate the standard fixed calendar from historical or otherwise distinct methods of establishing dates.
Applicable Hebrew Calendar Regime
Record the intended regime and distinguish reconstruction of an attested date from retrospective application of fixed rules.
- Does this use the standard fixed rabbinic calendar, an identified observational practice, or another specified Hebrew calendar tradition? definition
- What evidence supports applying that regime to the community and period being represented? provenance
Rule Sources and Calculation Authority
Establish which sources and implementations support the calendar's operational rules.
Traceable Calendar Rules
Associate leap-year, molad, postponement, and month-length rules with identifiable sources and implementation conventions.
- Which source specifies the leap-year cycle, molad arithmetic, and postponement rules used here? provenance
- When a calendar table and an implementation disagree, what evidence or designated authority determines which result the agent may use? action
Hebrew Year and Month Structure Represent the interdependent rules that determine year identity, intercalation, month order, and valid dates.
Hebrew date validity depends on the particular year's structure rather than a single permanent list of month lengths.
Year Numbering and Intercalation
Resolve year expressions and establish the applicable leap-year status.
Resolved Year and Cycle Position
Record the era, full year number, Tishrei transition, and cycle convention needed to determine intercalation.
- What full year and era does the expression denote, particularly when thousands are omitted? definition
- Under the selected fixed-calendar convention, what is the year's position in the 19-year cycle and is it common or leap? measurement
Month Identity and Year Shape
Resolve month names and derive the valid sequence and lengths for a particular year.
Valid Month and Day Combination
Distinguish Nisan-based numbering from Tishrei-based ordering, resolve Adar identity, and check dates against year-specific month lengths.
- Which named month does the supplied label or number identify, and does an unqualified Adar require clarification in this year? definition
- Do the calculated year length and Cheshvan and Kislev lengths permit this month-and-day combination? measurement
Day Boundaries and Date Conversion Control how Hebrew dates relate to local evenings, civil dates, and precise instants.
A civil date and a Hebrew date overlap across different boundaries, so conversion can be ambiguous even when calendar arithmetic is correct.
Local Evening Boundary
Capture the geographic and interpretive context required near the transition between Hebrew dates.
Resolved or Ambiguous Day Assignment
Record location and boundary conventions, including whether twilight leaves a date assignment unresolved for the intended use.
- Which location, time zone, and solar-boundary calculation apply to this instant? measurement
- Does the instant fall in a twilight interval or exceptional geographic situation for which the intended use requires a specified convention? boundary
Conversion Semantics
Distinguish conversion of date labels from conversion of timed events and historical date assertions.
Explicit Conversion Contract
Record the input's precision, the other calendar's identity, and whether the result denotes a conventional date correspondence or an interval of instants.
- Is the input a Hebrew date label, a local civil date, or a timestamp, and is the other calendar Gregorian, Julian, or explicitly something else? definition
- May the agent return one corresponding date, or must it return an interval or alternatives because time, location, or historical convention is missing? action
Hebrew-Date Scheduling and Observance Interfaces Support recurring dates and calendar-derived observance schedules while identifying conventions that require external authority.
Hebrew-date recurrence and observance scheduling depend on Adar choices, absent month days, and community-specific rules that date arithmetic alone cannot resolve.
Adar and Missing-Day Recurrence
Make recurrence policies explicit when the target year differs structurally from the original year.
Authorized Hebrew Anniversary Policy
Record the purpose and governing policy for moving an anniversary between common and leap years or into a month lacking the original day.
- For this recurrence's purpose, which Adar is used when moving between common and leap years, and who or what supplies that policy? provenance
- If the original day does not exist in the target month, should the agent apply a specified adjustment, omit the occurrence, or leave it unresolved? action
Observance Date Conventions
Separate nominal Hebrew dates from scheduled observance dates and from religious instructions about those observances.
Calendar-Derived Observance Schedule
Associate observance-date calculations with the applicable community, Israel or diaspora convention, and any sourced date-adjustment rule.
- Which community and Israel or diaspora convention govern the requested observance schedule? definition
- Is the requested result a nominal calendar date or an adjusted observance date, and which sourced rule permits any adjustment? boundary
Evidence and external alignment What the world already says about this thing, gathered so the model can be checked against it.
A model that cannot be lined up against existing standards, identifiers and practice cannot be adopted by anyone who already uses them.
Reported evidence
Findings from the breadth pass, kept separate from the structural claims.
Kinds and varieties
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Fixed arithmetic rabbinic calendar (the calculated luach in general use since late antiquity / the early medieval period)
- Observational court calendar (months sanctified by crescent sighting, as in the Temple and Sanhedrin era)
- Karaite calendar (still observation-based and not bound to rabbinic postponements)
- Common 12-month year versus embolismic 13-month leap year
- Deficient, regular, and complete year lengths (353/354/355 or 383/384/385 days)
- Tishrei-start civil and festival year versus Nisan-start biblical/regnal year
- Israeli statutory holiday calendar (one-day festivals, except Rosh Hashanah) versus diaspora liturgical calendar (yom tov sheni)
- Which of these kinds and varieties hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Identifiers and schemes
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Wikidata - Q41487 - Item for the Hebrew (Jewish) calendar as a calendar system.
- Unicode CLDR / BCP 47 calendar - hebrew (u-ca-hebrew) - Unicode calendar identifier used in locale tags and CLDR/ICU.
- Microsoft .NET calendar type - System.Globalization.HebrewCalendar; CAL_HEBREW = 8 - Windows/CLR civil calendar implementation of the calculated rabbinic calendar.
- Anno Mundi year number - AM n (currently 5786 as of 2026-09-07, becoming 5787 at 1 Tishrei) - Era count from the conventional epoch of creation, not a catalogue code.
- Which of these identifiers and schemes hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Standards and regulation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Rabbinic calculated calendar as codified in Maimonides, Mishneh Torah, Hilchot Kiddush HaChodesh (not a modern standards body, but the operative arithmetic source).
- Talmud Bavli, tractate Rosh Hashanah, and Shulchan Aruch, Orach Chaim - halakhic rules for months, festivals, and when the date changes (sunset).
- Hours of Work and Rest Law, 5711-1951 (Israel) - Shabbat and Jewish festivals as public rest days, dated on the Hebrew calendar.
- Israeli civil status, population registry, and rabbinical-court practice - Hebrew dates on official documents and for Jewish personal-status law.
- Unicode Technical Standard #35 (Unicode Consortium) - locale calendar type hebrew for software representation, not for the religious rules themselves.
- Which of these standards and regulation hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Real-world use
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Sets the dates of Shabbat-adjacent festivals, fasts, and the weekly Torah reading cycle in synagogues worldwide.
- Printed and digital luachot (wall, pocket, and app calendars) used in Jewish homes, schools, and Israeli public life, usually showing Gregorian and Hebrew dates together.
- Israeli government, schools, banks, and media announce public holidays and many events by Hebrew date (e.g. 5 Iyar Independence Day, 10 Tishrei Yom Kippur).
- Ketubot, gravestones, yahrzeit records, bar/bat mitzvah invitations, and many Jewish legal documents are dated in AM years and Hebrew months.
- Software localization (CLDR/ICU, phone calendars, .NET HebrewCalendar) converts civil dates for Jewish users and for Israeli holiday tables.
- Which of these real-world use hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Typical measurements
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Mean month (molad interval) - 29 days, 12 hours, 793 chalakim (exactly that conventional mean) - day (and chelek)
- Hour subdivision - 1080 chalakim per hour; 1 chelek = 10/3 seconds - chelek
- Month length - 29 or 30 - day
- Common year length - 353, 354, or 355 - day
- Leap year length - 383, 384, or 385 - day
- Intercalary cycle - 235 months in 19 years; leap years 3, 6, 8, 11, 14, 17, 19 of the cycle - month / year
- Anno Mundi year number - about 5786-5787 in 2026 CE; epoch near 3761/3760 BCE - AM year
- Which of these typical measurements hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Failure modes and hazards
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- The mean molad slowly diverges from the true astronomical conjunction, so over millennia seasons and lunar phase will drift relative to the fixed arithmetic calendar.
- Software that assumes twelve months, midnight date change, or Gregorian leap rules mis-converts Hebrew dates, especially around Rosh Hashanah, Cheshvan/Kislev, and Adar.
- Leap-year anniversary errors: birthdays, yahrzeits, and bar mitzvah calculations in Adar I versus Adar II are a recurring communal and software failure.
- Sunset-to-sunset day boundary (and disputed nightfall definitions) makes the 'current' Hebrew date depend on location and custom, unlike civil midnight.
- Diaspora second-day festival (yom tov sheni) versus Israeli one-day practice causes travel, calendar-app, and food-preparation mistakes.
- Postponement rules (deḥiyyot) make 1 Tishrei not always fall on the molad weekday, which people treat as a bug rather than a rule.
- Which of these failure modes and hazards hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Regional variation
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Israel observes biblical festivals for one day (Rosh Hashanah two days everywhere); the diaspora adds a second festival day.
- Hebrew month-name pronunciation and Latin spelling differ by community (Tishrei/Tishri, Cheshvan/Marcheshvan, Tevet/Tebeth).
- Nightfall used to close the date (degrees below horizon, 72 minutes, 42 minutes, and other customs) varies by community and latitude.
- Karaite communities (Israel, former Soviet Union, diaspora) reject the calculated rabbinic luach and sight the new moon.
- Reform and some liberal communities often drop yom tov sheni outside Israel; Orthodox diaspora practice retains it.
- Israeli public documents routinely carry a Hebrew date; most other states do not.
- Which of these regional variation hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Neighbouring kinds and how to tell them apart
Reported by the breadth pass; each item needs checking against its source before it becomes normative.
- Islamic (Hijri) calendar - Purely lunar with no leap month, so it slips through the seasons; the Hebrew calendar intercalates to keep Nisan in spring.
- Gregorian calendar - Solar civil calendar with midnight day boundary and February leap days; the Hebrew calendar is lunisolar, changes date at sunset, and leap-inserts a month, not a day.
- Chinese lunisolar calendar - Also Metonic-related and lunisolar, but uses different epoch, leap-month placement, solar terms, and month numbering; it is not the Jewish festival calendar.
- Samaritan calendar - Related Levantine lunisolar practice with its own epoch, sighting/calculation rules, and festivals; not the rabbinic luach.
- Babylonian calendar - Historical ancestor sharing month names (Nisanu, Tammuz, Elul, etc.) and lunisolar intercalation, but not Anno Mundi numbering or deḥiyyot.
- Julian calendar - Solar Christian/civil calendar of 365.25-day mean year; used in some churches and in historical Jewish-civil conversions, not for Jewish months.
- Which of these neighbouring kinds and how to tell them apart hold for the sense of Hebrew calendar this model covers, and on what evidence? provenance
Sources
- Hebrew calendar - Wikipedia (Wikimedia Foundation) - Structure of the lunisolar year, month names, 19-year intercalary cycle, postponement rules, Anno Mundi epoch, and the distinction between calculated and observational practice.
- Calendrical Calculations - Nachum Dershowitz and Edward M. Reingold, Cambridge University Press - Canonical computational statement of molad arithmetic, year-length classes, conversion to and from the Gregorian calendar, and the mean lunation of 29 days 12 hours 793 chalakim.
- Mishneh Torah, Hilchot Kiddush HaChodesh - Moses Maimonides (standard printed and Sefaria texts) - Classical rabbinic formulation of sanctification of the month, intercalation, and the calculated calendar's mean-motion parameters.
- Unicode Technical Standard #35 (LDML) - Unicode Consortium - Machine identifier hebrew / BCP 47 u-ca-hebrew used in locale data, ICU, and software calendars.
What the second pass must settle
- Does the registry intend this entry to own only the standard fixed rabbinic calendar or also historical and other Hebrew calendar regimes?
- Which authoritative sources and independent reference calculations should establish the supported fixed-calendar rules and conversion range?
- For which historical periods can date assignments be supported as attested practice, and where must results be labeled retrospective or uncertain?
- Which conventions should govern twilight, high-latitude conditions, and geographic date-boundary disputes for the intended applications?
- Which recurrence and observance-date policies may this model apply directly, and which require a linked community or religious-authority model?