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The civil Hebrew New Year begins on the first of Tishri, which occurs in the fall during September or October. The molad, an astronomical phenomenon, is the Hebrew word for the conjunction of the earth, moon, and sun. The first of Tishri does not always occur on the day of the molad of Tishri because of dehioth (postponements). When the molad of Tishri occurs at a time unaffected by the postponement rules Tishri 1 is on the same day as the molad. The four postponement rules are:

1. The occurrence of the molad of Tishri at 12 noon (18 hours 0 parts) or later postpones Tishri 1 to the next day because the crescent of the moon must occur on Tishri 1 under the postponement rules. Although a Hebrew day began at sunset, for the purpose of computing the conjunctions of the sun and moon, the days commenced at the preceding noon. This means that the official day for calculating a molad goes from noon to noon and not from sunset to sunset. Of course, causing the day to fall on a Monday, Wednesday, or Friday, due to a postponement delays it for an additional day. A delay of a molad can not be for longer than two days nor advanced forward.

2. The first of Tishri may never occur on a Sunday, Wednesday, or Friday. This rule prevents the occurrence of two consecutive (back-to-back) sabbaths during the fall religious festival season. If the molad of Tishri, or a postponement, falls on a Sunday, Wednesday, or Friday, then Tishri 1 falls one day later, to a Monday, Thursday, or Sabbath, respectively. Such delays are known as political postponements. Two successive sabbath days would create some inconvenience in the general life of the community. Recall that sabbaths include both weekly and annual sabbaths (the high sabbaths or seven annual holy days).

3. Since the seventh-day sabbath is a Saturday a postponement is necessary when the molad of Tishri falls on a Sunday or a Friday in order to avoid two consecutive sabbaths (a weekly sabbath and a fall holy day) back-to-back). This avoids causing the preparation days for the Feast of Trumpets, the Feast of Tabernacles, and the Feast of the Great Day from being weekly sabbaths and the preparation day for the weekly sabbath being the Feast of Trumpets or the Feat of the Great Day. A Friday postponement also prevents the Day of Atonement from falling on a Sunday avoiding two consecutive sabbaths.

4. The first of Tishri may not occur on a Wednesday to block the Day of Atonement (Tishri 10), a high sabbath, from falling on a Friday. A Friday Day of Atonement would cause particular hardship for the community since food preparation could not occur on a weekly sabbath day even though it followed a required fast day.

The occurrence of the molad of Tishri of a common year falling 3 hours 204 parts or later (after 3:12 a.m.) on a Tuesday, requires the delay of the first of Tishri until Wednesday, and by the first rule further postponed to Thursday. This delay prevents the year commencing with that molad of Tishri from having more than 355 days, which is not allowed, and still permit the common year that ends with this molad of Tishri to have 354 days. A delay until Thursday instead of Wednesday arises because Wednesday may never be the first of Tishri under the first rule cited above.

A molad of Tishri occurring on or after 9 hours 589 parts or later (that is, 9:33 a.m. or later) on a Monday in a common year immediately following an intercalary year requires delay of the first of Tishri until Tuesday. This delay results from the embolistic year before having started with a Thursday. The common year, the year that begins with the molad of Tishri in question, must start on a Tuesday, Wednesday, or Thursday. Tuesday is the only permissible day because the first rule cited above one eliminates Wednesday and since no postponement may be greater than two days eliminates Thursday.

Under the rules of the Rabbinic Calendar, Passover regulates all the other festival times in the Hebrew calendar. Passover Sabbath, Nisan 15, begins at twilight at the end of the 14th day of the first month (Leviticus 23:5). The Hebrew day extends from sunset to sunset. Passover Sabbath (the first high day feast of the days of unleavened bread) always occurs with a full moon rising in the east, the 15th day after the new moon. It could not occur before the vernal equinox. By Rabbinical rules Passover can never occur on a Monday, Wednesday, or Friday. The day starting at the previous sunset. Preventing Passover Sabbath from falling on Monday, Wednesday, or Friday eliminates the possibility that Tishri 1 would occur on a Friday, Sunday, or Wednesday. This is because Tishri 1 is always 163 days after Nisan 15. Succoth, the Feast of Tabernacles, was also set to begin on a full moon, the fifteenth day of the first month, Tishri (Leviticus 23:34). Succoth must also occur in the fall, after the gathering of crops (Deuteronomy 16:16).

The Calculated Hebrew Calendar, which we refer to as the Jewish Reformed Calendar but also called the Sacred Calendar, is a mathematical model simulating the orbital periods of the sun, moon, and earth from the perspective of the earth. It takes advantage of the Metonic cycle, or 19-year cycle, wherein 19 Hebrew years (12 common years and 7 leap years) or 235 Hebrew months is 1 hour 485 parts less than 19 solar years. The average Hebrew year length is 365.2468 days, in contrast to the mean tropical solar year which is measured at roughly 365.2425 days (365 days 5 hours 48 minutes and 46 seconds). Approximately every 216 years, the Hebrew year is "slower" than the average solar year by a full day. In order to correct for this difference one must add an additional day to the Hebrew Calendar after each 222 years. Since the average Gregorian year is 365.2425 days and repeats every 400 years, the average Hebrew year is slower by a day every 231 Gregorian years.

The Julian solar year was exactly 365 days 6 hours and 0 seconds in length. This is a little over 11 minutes too long in relation to a mean tropical solar year. The Hebrew year is shorter and precedes the Julian solar year by 10 d 21 h 204 p.

The calendar is simply a mathematical model based on averages wherein 235 lunar months (synodic months) exceed 19 solar years (tropical years) by about 2 hours (actually, 0.0812 days or 1 hour 56 minutes 55.7 seconds. It is not a perfect mathematical emulation of physical reality. The Calculated Calendar is, however, a highly accurate approximation and it provides a statistically certain level of confidence. Nevertheless there is still a small standard error in measurement.

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