About the Hebrew Calendar
The Hebrew calendar is a lunar calendar based on 19-year cycles. The following sections explain the principles of this calendar. The relationship of the Hebrew year to the Civil year is illustrated by this chart. Units of Time
The calculations in the subsequent sections are based on an ancient form of measuring time. Each day is divided into 24 hours, which are each divided into 1080 parts (or "halakhim").
A month is defined as the period from one new moon to the next, corresponding to the astronomical definition of a siderial month. For these calculations the length of a month is taken to be 29 days, 12 hours, and 793 halakhim.
The Biblical date of creation is used as the basis for the Hebrew calendar. Reverse calculation gives the time of the first new moon of Year 1 as 5 hours and 204 halakhim from the beginning of the second day of the week (Monday). This date corresponds to October 7, 3761 BCE, using the Julian (not Gregorian) calendar. For later arithmetic convenience, we will define the preceding Sabbath as day number 0, so that this date, 1 Tishrei of Year 1, is day number 2. Types of Years
Kitchen Calendar Magnets
A year in the Hebrew calendar is normally twelve months: Tishrei, Cheshvan, Kislev, Tevet, Sh'vat, Adar, Nisan, Iyar, Sivan, Tammuz, Av, and Elul. An extra month (Adar is replaced with Adar I and Adar II) is intercalated in seven of every nineteen years (years 3, 6, 8, 11, 14, 17, and 19). Every 19-year cycle thus contains 235 months. These two cycles of years and months are so close that they diverge at the rate of only 4.5 days every 1000 years.
Calendar Magnets Because a lunar month is not a whole number of days, the length of the months can be either 29 or 30 days. A typical year alternates 30- and 29-day months, beginning with Tishrei, which is 30 days. In leap years, Adar I is 30 days and Adar II is 29 days. To further bring the calendar into alignment with the lunar cycle, a year may need to be shortened or lengthened by one day: Kislev may be shortened to 29 days, or Cheshvan may be lengthened to 30 days. Thus, there are six types of years:
* Defective common years, with 353 days
* Regular common years, with 354 days
* Excessive common years, with 355 days
* Defective leap years, with 383 days
* Regular leap years, with 384 days
* Excessive leap years, with 385 days