Calendar FAQ v1.6 -- Introduction

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Copyright and disclaimer
        This document is Copyright (C) 1996 by Claus Tondering.
        The document may be freely distributed, provided this
        copyright notice is included and no money is charged for
        the document.

        This document is provided "as is". No warranties are made as
        to its correctness.

        This is the calendar FAQ. Its purpose is to give an overview
        of the Christian, Hebrew, and Islamic calendars in common
        use. It will provide a historical background for the Christian
        calendar, plus an overview of the French Revolutionary
        calendar and the Maya calendar.

        Comments are very welcome. My e-mail address is given above.

        I would like to thank
              - Dr. Monzur Ahmed of the University of Birmingham, UK,
              - Michael J Appel,
              - Jay Ball,
              - Chris Carrier,
              - Simon Cassidy,
              - Claus Dobesch,
              - Leofranc Holford-Strevens,
              - H. Koenig,
              - Marcos Montes,
              - James E. Morrison,
              - Waleed A. Muhanna of the Fisher College of Business,
                Columbus, Ohio, USA,
              - Paul Schlyter of the Swedish Amateur Astronomer's Society
        for their help with this document.

Changes since version 1.5
        A few of minor corrections.
        Section 1.1 about solstices and equinoxes added.
        More details given in the Easter algorithm of section 2.9.6.
        Section 5 about the week has beed added and the following
        sections renumbered.

Writing dates and years
        Dates will be written in the British format (1 January)
        rather than the American format (January 1). Dates will
        occasionally be abbreviated: "1 Jan" rather than "1 January".

        Years before and after the "official" birth year of Christ
        will be written "45 BC" or "AD 1997", respectively. I prefer
        this notation over the secular "45 B.C.E." and "1997 C.E."

The % operator
        Throughout this document the operator % will be used to
        signify the modulo or remainder operator. For example, 17%7=3
        because the result of the division 17/7 is 2 with a remainder
        of 3.

The text in square brackets
        Square brackets [like this] identify information that I am
        unsure about and about which I would like more
        information. Please write me at

The Calendar FAQ v.1.6 consists of three parts:

In Part 1: 1. What astronomical events form the basis of calendars? 1.1. What are Equinoxes and Solstices? 2. The Christian calendar 2.1. What is the Julian calendar? 2.1.1. What years are leap years? 2.1.2. What consequences did the use of the Julian calendar have? 2.2. What is the Gregorian calendar? 2.2.1. What years are leap years? 2.2.2. Isn't there a 4000-year rule? 2.2.3. Don't the Greek do it differently? 2.2.4. When did country X change from the Julian to the Gregorian calendar? 2.3. What day is the leap day? 2.4. What is the Solar Cycle? 2.5. What day of the week was 2 August 1953? 2.6. What is the Roman calendar? 2.6.1. How did the Romans number days? 2.7. Has the year always started on 1 January? 2.8. What is the origin of the names of the months? In Part 2: 2.9. What is Easter? 2.9.1. When is Easter? (Short answer) 2.9.2. When is Easter? (Long answer) 2.9.3. What is the Golden Number? 2.9.4. What is the Epact? 2.9.5. How does one calculate Easter then? 2.9.6. Isn't there a simpler way to calculate Easter? 2.9.7. Is there a simple relationship between two consecutive Easters? 2.9.8. How frequently are the dates for Easter repeated? 2.9.9. What about Greek Easter? 2.10. How does one count years? 2.10.1. Was Jesus born in the year 0? 2.10.2. When does the 21st century start? 2.11. What is the Indiction? 2.12. What is the Julian period? 2.12.1. What is the modified Julian day? 3. The Hebrew Calendar 3.1. What does a Hebrew year look like? 3.2. What years are leap years? 3.3. What years are deficient, regular, and complete? 3.4. When is New Year's day? 3.5. When does a Hebrew day begin? 3.6. When does a Hebrew year begin? 3.7. When is the new moon? 3.8. How does one count years? 4. The Islamic Calendar 4.1. What does an Islamic year look like? 4.2. So you can't print an Islamic calendar in advance? 4.3. How does one count years? In Part 3: 5. The Week 5.1. What Is the Origin of the 7-Day Week? 5.2. What Do the Names of the Days of the Week Mean? 5.3. Has the 7-Day Week Cycle Ever Been Interrupted? 5.4. Which Day is the Day of Rest? 5.5. What Is the First Day of the Week? 5.6. What Is the Week Number? 5.7. Do Weeks of Different Lengths Exist? 6. The French Revolutionary Calendar 6.1. What does a Republican year look like? 6.2. How does one count years? 6.3. What years are leap years? 6.4. How does one convert a Republican date to a Gregorian one? 7. The Maya Calendar 7.1. What is the Long Count? 7.1.1. When did the Long Count Start? 7.2. What is the Tzolkin? 7.2.1. When did the Tzolkin Start? 7.3. What is the Haab? 7.3.1. When did the Haab Start? 7.4. Did the Maya Think a Year Was 365 Days? 8. Date

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