The Jewish Calendar - Section 4: Determining Month Offsets and Leap Years

Let's gather some useful information on leap year timing and lunar month calculations. The size of a 19-year block (Jewish regular/leap year cycle) will come in handy here. Such a cycle contains 12 regular years and 7 leap years. Since each regular year contains 12 months (and hence lunar cycles) and each leap year 13, the total number of lunar cycles for a 19-year block is:

Dividing this by 19 yields the average size of a year, 12 7/19 lunar months. The 235-month block amounts, in days (with the partial day expressed in hours, minutes and seconds), to:

The offset, in months, from the beginning of a 19-year block can be determined for the beginning of each year within that block. Here is a table that shows two kinds of offsets:

Year # (position in block)	Classification	Month offsets based on:
Year # (position in block)	Classification	actual year sizes	average year size
1	Regular	0	0
2	Regular	12	12 7/19
3	Leap	24	24 14/19
4	Regular	37	37 2/19
5	Regular	49	49 9/19
6	Leap	61	61 16/19
7	Regular	74	74 4/19
8	Leap	86	86 11/19
9	Regular	99	98 18/19
10	Regular	111	111 6/19
11	Leap	123	123 13/19
12	Regular	136	136 1/19
13	Regular	148	148 8/19
14	Leap	160	160 15/19
15	Regular	173	173 3/19
16	Regular	185	185 10/19
17	Leap	197	197 17/19
18	Regular	210	210 5/19
19	Leap	222	222 12/19

TABLE 4. Month offsets of the years in a Jewish 19-year block.

Notice what happens when you chop off the fractional part of each average-based month offset. You get the same number as the actual-based offset for each row—except, unfortunately, for year #9! (Why this year wasn't chosen to be a leap year instead of #8, I don't know.) It would have been nice to be able to use a formula such as "actual month offset = INT ([year - 1] × [12 + 7/19])", where [year - 1] × [12 + 7/19] is the average-based month offset (remember, the average year size is 12 7/19 months). But don't despair! We can "nudge" our way out of this problem by "nudging" up the number from the average-based offset by 1/19. As a result of this, the integer part of this adjusted offset remains unchanged for each year—except for year #9. Adding 1/19 to 98 18/19 in this year's row yields 99 (which, of course, remains intact even after applying the integer operation, because there is no fractional part to chop off at this point). This matches the corresponding actual-based month offset! (Now we're getting somewhere!) We can now use a formula like "actual month offset = INT (([year - 1] × [12 + 7/19]) + 1/19)". But it is important to avoid decimal rounding with this particular formula in the year #9 case, because such rounding resulting from division by 19 may lead to a number just a "teensy-weensy" bit under 99 immediately before the integer ("INT") operation, resulting in an erroneous 98. Let us re-evaluate the formula to a safer form, using y for the year:

This ensures that the numerator will evaluate in a year #9 case to a number that is evenly divisible by 19, thus suppressing any rounding errors ([235y - 234]/19 = [2,115 - 234]/19 = 1881/19 = exactly 99). Reasonable rounding in situations involving any of the 18 other year cases should not cause any problems.

So now we have a convenient formula that we can use to calculate actual month offsets for the years within a 19-year block, from the start of that block. But what about also having a similarly convenient formula that, given any Jewish year, will give us the number of lunar cycles from the very first Rosh Hashanah's molad to that of the given year (i.e., the actual-based month offset from the molad of Tishri of the year 1)? The above formula will obviously work for the first 19 Jewish years, but what about the later ones?

Let us gather some information on the Jewish year 20, using 19, a year whose offsets we already have. Since 20 is the first year in a block of 19, it is a non-leap year, just like the year 1. Since 19 is a leap year, the actual-based month offset for 20 is 235 (i.e., 222 + 13). The average-based offset is also 235 (i.e., 222 12/19 + 12 7/19). And 235 months is the exact size of a 19-year block. Just as the actual-based and average-based offsets coincide exactly for the year 1, they do so for 20 as well. This coincidence also occurs for every 19th year afterward. Hence, the difference between the average-based and actual-based offsets for the second year in any 19-year block will always be 7/19, the difference between the month offsets for the third year in any such block will always be 14/19, the difference for the fourth year will always be 2/19, and so on. Alas, the difference for the ninth year will always be -1/19. But after taking the "nudging" addition of 1/19 into account, the difference for any year in a block of 19 is still going to match the difference of the same positional year in any other block. The difference between the nudged average offset and the actual will always be 0 for the ninth year, 1/19 for the first year, 8/19 for the second, 15/19 for the third, 3/19 for the fourth, etc., no matter which 19-year block is used. Keep in mind that these differences also happen to be the fractional parts of the nudged average offsets. Therefore, truncating such a fractional part will work every time—you'll always get the corresponding actual-based month offset! Need I say more? Let me illustrate by using a table of sample years:

Jewish year	Block #	Positional year # in block	Classification	Month offsets based on:
Jewish year	Block #	Positional year # in block	Classification	actual year sizes	average year size	plus nudge
1	1	1	Regular	0	0	1/19
2	1	2	Regular	12	12 7/19	12 8/19
3	1	3	Leap	24	24 14/19	24 15/19
4	1	4	Regular	37	37 2/19	37 3/19
9	1	9	Regular	99	98 18/19	99
17	1	17	Leap	197	197 17/19	197 18/19
19	1	19	Leap	222	222 12/19	222 13/19
20	2	1	Regular	235	235	235 1/19
21	2	2	Regular	247	247 7/19	247 8/19
22	2	3	Leap	259	259 14/19	259 15/19
23	2	4	Regular	272	272 2/19	272 3/19
28	2	9	Regular	334	333 18/19	334
36	2	17	Leap	432	432 17/19	432 18/19
38	2	19	Leap	457	457 12/19	457 13/19
39	3	1	Regular	470	470	470 1/19
40	3	2	Regular	482	482 7/19	482 8/19
47	3	9	Regular	569	568 18/19	569
5757	303	19	Leap	71,192	71,192 12/19	71,192 13/19
5758	304	1	Regular	71,205	71,205	71,205 1/19
5759	304	2	Regular	71,217	71,217 7/19	71,217 8/19
5766	304	9	Regular	71,304	71,303 18/19	71,304

TABLE 5. Month offset analysis of sample years.

So the month offset formula that works for any year within a 19-year block from the start of that block should also work for any year, no matter which block it is in, from the molad of the very first Tishri. In fact, the same formula can be implemented for offset calculations from the beginning of any 19-year block for a later year beyond that block (all that is needed is a small modification of the input value y)! As shown earlier, INT ((235y - 234)/19) is derived from INT (([y - 1] × [12 + 7/19]) + 1/19), which is a restatement of "the integer of (([year - 1] × [average year size]) + nudge)". To compute the actual-based month offset of a given year from the start of an earlier 19-year block, the first year of that block is subtracted from the given year. But before this result is plugged into the formula, a further adjustment is made by adding 1. Afterwards y can be set to this new result, and we're ready to compute the offset.

Alternatively, we can compose a more direct formula for this kind of offset. If b is the beginning of a block, and y is a later year, then:

In fact, this formula can be used to compute the offset from the start of year 1. That is, If b = 1, then

And this formula can also be used to compute the offset within a single 19-year block, when b = 1 and y = the positional year number in that block.

So at last we have a useful formula to help us determine the number of actual Jewish calendar months, or lunar cycles, from the beginning of any block (whether the very first one or later) to the start of any year that occurs afterward (whether it's in that same block or not)!

Given the first year of any 19 Jewish year block, and any later year (whether within the same block or not):

The number of Jewish calendar months (lunar cycles), from the beginning of that first year in the block to the beginning of the later year is:

INT (([[y - b] × 235] + 1)/19)

where y is the later year, and b is the block's first year.

Since there are an overwhelming amount of cases that involve computing this offset from the start of Jewish year 1, I am presenting this more specific, first-year formula as well:

Given any Jewish year (1 or later):

The number of Jewish calendar months (lunar cycles), from the beginning of year 1 to the beginning of that given year is:

INT ((235y - 234)/19)

where y is the given year.

I expect to be using primarily this latter formula rather than the former, for the rest of this discussion.

As mentioned earlier, a given year's positional number in a 19-year Jewish block can be determined by the remainder when that year is divided by 19 (except that if the remainder is 0, then the positional number is 19). The year is a leap year if its positional number is 3, 6, 8, 11, 14, 17 or 19. If its positional number is anything else, the year is regular one.

There is another, and rather clever, way to determine a year's leap status. Let's take another look at the nudged (i.e., with 1/19 added) average-based month offsets. Upon studying these offsets closely (take a good look at the month offset tables), it shouldn't take much effort to realize that the fractional part for each year's such offset always contains 19 in the denominator, and some kind of lesser number in the numerator. As for a year whose positional year number in a block is 9, the offset does not have a fractional part, but at least for now let us assume "0/19" as such a part for that year (picture the offset as being stated, for example, as "99 0/19" or "334 0/19").

What these nudged offsets show are the exact results of ([y - 1] × [12 + 7/19]) + 1/19, whose equivalent is (235y - 234)/19, for each year y. In other words, this is what you get when you divide 235y - 234 by 19. Now when you divide a number by 19, one of the ways to express the result is with the exact number, which can include a fractional part expressed in nineteenths. The denominator is always 19, but the numerator can vary, depending on the result. But another way to express the division result is with the integer quotient and a remainder. This remainder is, of course, the same number that appears as the numerator in the fractional part of the corresponding exact result! (Don't forget that the remainder is always 0 when positional year #9 is used.)

There is an interesting point here: if this remainder is 12 or more, the year is a leap year, but if the remainder is 11 or less, the year is a non-leap year! (Go ahead, look at the tables again, and check it yourself! Is this neat or what?)

Let y = any given Jewish year (1 or later):

If the remainder, upon dividing

235y - 234

by 19, is 12 or greater, then the given year is a leap year. Otherwise, the year is a regular one.

But wait—there's more! Based upon the information from the tables, have you noticed that the remainder from dividing 235y - 234 by 19 for a given year typically changes by 7 for an adjacent year? The remainders for the 19 positional years in a block (starting from the first year) are 1, 8, 15, 3, 10, 17, 5, 12, 0, 7, 14, 2, 9, 16, 4, 11, 18, 6 and 13. The spacing would be 7 for every adjacent year if, after counting by 1 upwards to 18, the counting wrapped back around to 0. What this means is that if 7 is added to the remainder for a given year, you will get the remainder for the next year as long as the sum does not exceed 18. But if it does, just subtract 19 from it in order to get that next year's remainder. Conversely, if 7 is subtracted from the remainder for a given year, you will get the remainder for the previous year as long as the result is not negative. But if it is, just add 19 to it (i.e., subtract the absolute value of the negative number from 19) in order to get that previous year's remainder. I'd like to refer to this kind of math as rotational addition or subtraction of 7 based on a range of 19 numbers, from 0 to 18. This can come in handy if we need to check the leap status of an adjacent year in accordance with whether or not its corresponding remainder is 12 or greater (given the same kind of remainder for the year that we were working with in the first place).

All we need at this point is the exact time of an actual new moon, and we'll be on our way to determining a calendar for a Jewish year!

Okay, so what's a good molad reference point? A very popular example is the molad of Tishri of the Jewish year 1, the new moon corresponding to the very first Rosh Hashanah (at least as tradition puts it). Also known as the BaHaRaD—a term that uses Hebrew letters whose numerical values describe the time, in modern Jewish notation, as 2 (Bet) days (or Monday), 5 (Hey) hours and 204 (Resh-Dalet) parts—the corresponding time in Western notation is Sunday night at twenty seconds after 11:11 p.m. (at least one source puts the corresponding Gregorian date at the 6th of September in the year 3760 BCE, assuming that an intervening year 0 occurs between 1 BCE and 1 CE).