FAQ Frequently Asked Questions

[Note. Questions may be added to this section from time to time. I hope the information here is accurate. If you detect errors, please write to me, corre@uwm.edu Last update 2004:08:27.]

1. I wanted to find the date of Yom Kippur (Day of Atonement) in 1986. I entered +1986 at the prompt, but it did not give me the date in 1986. Why is this?

You probably know that the Jewish and civil calendars overlap. When you enter +1986, you get a calendar for the Jewish year beginning in 1985, most of which is in 1986. If you want the full year 1986, enter +1986, leave a space and follow it by +1987. In this way you get the full civil year that is covered by the two adjacent Jewish years. To sum up, to get a full civil year enter the year (preceded by a plus sign) followed by a space and the next year also preceded by a plus sign.

2. My grandmother was born in 1906, but does not know the exact date of her birth. My aunt says grandmother was born two days before Purim. What is the civil date?

Enter +1906 at the prompt in the program. Then look for the month Adar, locate the fourteenth of the month, and count back two days. Then locate the corresponding civil date on the other side of the screen. If it is a leap year, you must look for the second Adar. You know it is a leap year by looking at the status line. If the Jewish year has more than 380 days, it is a leap year. If you are looking for a date early in the Jewish year, which is the latter part of the civil year, you must enter the next year along, (+1907) to get the right part of the year. This calendar works on the basis of the Jewish year, and begins in the fall, whereas the civil year begins in the winter. For your convenience here are the Jewish dates of the major festivals:

Rosh Hashanah (New Year)	Tishri 1-2
Yom Kippur (Atonement)	Tishri 10
Sukkot (Tabernacles)	Tishri 15-16
Hoshana Rabba	Tishri 21
Shemini Atseret	Tishri 22
Simhat Torah	Tishri 23
Hanukkah (First Day)	Kislev 25
New Year for Trees	Shevat 15
Purim	Adar 14
Passover (first days)	Nisan 15-16
Passover (last days)	Nisan 21-22
Holocaust (Shoah) Day	Nisan 27
Israel Independence Day	Iyyar 5
33rd day of the Omer	Iyyar 18
Shavuot (Pentecost)	Sivan 6-7
Fast of Av	Av 9

Please note the following:

• In Israel, Rosh Hashanah is observed for two days, but the other major holidays are observed for one day only.
• If Holocaust Day and Israel Independence Day occur on Friday or Saturday, they are advanced to the preceding Thursday.
• If the Ninth of Av falls on Saturday, the fast is observed on the following day, Sunday.
• Purim is observed one day later in Jerusalem, Bagdad, and some other old cities.
• In some years the month of Kislev has 29 days, and in others it has 30 days. In years when Kislev has 29 days, Hanukkah ends on Tevet 3. In years when Kislev has 30 days, Hanukkah ends on Tevet 2. Put another way: Tevet 2 is always part of Hanukkah; Tevet 3 is sometimes part of Hanukkah; Tevet 4 is never part of Hanukkah. I had to mention this because my eldest grandson was born on Tevet 3, so it is a unique day for me for two reasons.

3. When is Yizkor (memorial prayer) said?

On Tishri 10, Tishri 22, Nisan 22, Sivan 7. In Israel the dates are Tishri 10, Tishri 22, Nisan 21, Sivan 6.

4. Since the Jewish day starts in the evening, does your calendar equate the civil day to the Jewish day that starts that evening, or the day in which the daylight hours occur?

The second. So far as I know, all calendars that have Jewish and civil dates function this way. If a marriage takes place after dark, the Jewish date is the one corresponding to the next day on the civil calendar. Stay away from the twilight hour if you want to avoid doubt!

5. Why is the Jewish calendar so complicated?

Because it reconciles the sun year, which ensures that festivals fall around the same season, with the moon month, which calls for a New Moon at the beginning of the month. Since the precise length of the sun year, and the precise time between appearances of the New Moon are both rather awkward figures, a lot of calculating has to be done to achieve this. In addition, the calendar is modified to ensure that the Day of Atonement never falls on a Friday or a Sunday (which would create difficulties with respect to the adjacent sabbath) and that the seventh day of Tabernacles ("The Great Hosanna") does not fall on a sabbath, when it would be impossible to fulfil the ancient (and rather mysterious) custom of beating a bunch of willow leaves.

6. Where can I find further reading on the Jewish Calendar?

Try Nachum Dershowitz and Edward M. Reingold, Calendrical Calculations Cambridge University Press, 1997, Chapter 9. There is a reading list on p. 102.

7. Is it true that the Jewish calendar corresponds with the civil calendar again after nineteen years?

Would it were so simple. In fact there are factors in both calendars which prevent this from happening, even though there is an approximate equivalence every nineteen years. According to Dershowitz and Reingold, the Jewish calendar repeats only after 689,472 years.

8. Will your calendar compute any future year?

The algorithm used will compute any future year. But the architecture of the computer on which it is implemented, as well as limits on the size of integers in the programming language in which the program was written place practical limits. It seems unlikely that anyone would need such a distant date though.

9. I put in several dates, and nothing came out. Why?

Your data was probably bad. Only numeric characters can be entered, with a plus or minus sign immediately in front, if you want a civil year. Be careful not to confuse 0 (zero) with O (the letter after N) or 1 with l (lower case L). Also, you must have at least one space between the years you enter, even if the year is preceded by a plus or minus sign. It would be possible to modify the program to correct some of these errors, and in fact some correction is done, but Troutman's fifth law of programming declares that however cleverly you edit the incoming information, someone will always find a new way of creating problems, so why bother? It needs care on the part of the user.

10. Does your calendar have a year 2000 problem?

No. Years are referenced by all the digits they need, not just the last two.

11. I cannot see the civil months which are supposed to be on the right side of the screen. Why is this?

Your screen is too narrow. Decrease the size of the image if you can, by using the view feature, or something similar in your browser. Or move to the right side of the image. It should print out correctly on your printer.

12. Why is there no year 0 in your calendar?

Look at it this way. If we count back in the Gregorian Calendar 1999 years, [this was written in 2000] we get to the year 1 of the current era. That will be (mostly) 3761 in the Jewish Calendar. Neither of those figures was known to the people at that time. These numerations were figured out centuries later according to what people felt to be the birth of Jesus or the creation of the world. Scientists dispute both results, one by about four years, the other by millions. Accordingly it is totally arbitrary what we designate the year prior to 1AD. Traditionally it has been 1BC (-1) and I accepted this. But some scientists prefer to use 0 for that year, and if you accept this, my BCE dates are one year off. There is no right or wrong about this. It depends if you want a "scientific" zero, or to accept the fiction that that year is one year before Jesus' birth which means you go straight from -1 to +1.
This has some relevance to the silly question whether 2000 is or is not the first year of the millenium. If you call the year before 1 zero, – which is your privilege – you can designate it year 1 of the first century and millenium, and then 2000 is the first year of the new century and millenium. In Israel bathrooms are always designated 000, so if on a floor there are rooms 000, 001, 002 and 003, there are FOUR rooms on that floor. Get the picture?

The following comment raises a point which often crops up in different forms – ADC
13. I would like to point out one thing that you may find useful. You point out that because of the irregular intercalation in the early Roman period, the dates for those years are merely approximate. In fact, they represent a theoretical Gregorian calendar extended back into time. However, the Gregorian calendar began only on october 15, 1582 ce. The day before that was october 4 on the Julian calendar, still used by some Eastern Orthodox chr*stians. This means that the Gregorian Calendar began on 19 Tishrei, 5343. 18 Tishrei was october 4, and suddenly, the next day, it was october 15. Of course this applied originally only to some Roman Catholic countries. Britain and her American colonies didn't switch over until 6 Tishrei, 5513.
Thus, to extend the perpetual calendar more accurately into the past (keeping in mind that the original Roman calendar did not recognize the seven-day week, and that early intercalation of the Julian calendar was quite confused, as you said), the date before 19 Tishrei, 5343 (october 15, 1582) should be shown as the fourth, rather than the fourteenth. Also, up until that point ALL years divisible by four, including all centesimal years, were leap years. This is the difference between the Julian and Gregorian calendars, and the reason ten days had to be dropped in 1582 (and eleven days in 1752). In the Gregorian calendar, the leap year is skipped in centesimal years unless that year is divisible by 400 (e.g., 1600, 2000).
I realize you knew all this already, and that this e-mail will probably only skip back to me anyway, [It didn't. -- ADC] but I just wanted to show you what a calendar enthusiast I am and how much I appreciate your perpetual calendar on the Internet–er, despite the fact that I've never been able to do math very well!

My son agrees with you. He feels that I should have followed the Julian calendar until 1582, and then switched over to the calendar proclaimed by Pope Gregory XIII in that year. I wish to explain the reasons that I made the decision I did. It was just one of many required in creating a calendar of this type, but an important one.
The Julian calendar was introduced by Julius Caesar in 46 B.C.E. It was not very accurate, but better than what came before. Prior to that the Roman Republican Calendar had been in use. According to tradition this had been in use since around 738 B.C.E. Apparently it consisted of ten months; the months from September to December were the last four months of this ten-month calendar (septem=7, decem=10 etc.) The year consisted of only 304 days. What about the remaining sixty-odd days? They didn't worry about those. Things were pretty informal that long ago. Now let me ask: What am I supposed to do with the equivalences of the days from the first day of the first year of creation in the Jewish calendar? Why should they be equated with the inaccurate Julian calendar which was not even in use.
Now look at the other end. True, many people paid attention to the decree of Pope Gregory. But many did not. King Henry VIII of England, he of the many wives who died in 1547, had roundly declared that "The Bishop of Rome hath no power in this kingdom," and it had earned him excommunication, which worried him not at all. So the Gregorian calendar did not come to England until many years later, when the London mob went on a rampage convinced that they had been cheated of eleven days of life! My natural anglophilia makes me prefer Henry to Gregory, so why not switch when my native country switched? Or why not switch when the last major European power switched in 1917?
To me there seemed only one answer. The Gregorian calendar is a pretty accurate calendar. Let us use it throughout, even during times when something else existed, the Seleucid Era, the Roman Republican Calendar, the Julian Calendar. And let us equate it to the calculated Jewish calendar, whether or no Jews were calculating their calendar in those early days, or watching the signs of heaven and earth. After all, no one walking on this planet in 4 C.E. knew what year it was. That numbering system came years later. But we can legitimately use it.

The following question raises a point which often crops up in different forms – ADC
14. I found your Web Site regarding the dates very interesting. What are the chances that 14 Nisan in 30 AD occurred on a Friday? I am writing a book and need a little insight. I am under the impression that 14 Nisan occured on a Friday in both the years 30 and 33 AD. Is that possible? Thanks.

You can get the equivalence you want by typing in the year you want at the prompt. If you do not see the cursor flashing, click on the prompt. By entering +33 you will get the Jewish year that starts in the autumn of the previous year, 32. If you want the complete civil year 33 C.E. you must enter +33 +34, being careful to leave a space between the two entries. This is because the civil and Jewish years overlap, and by default the calendar gives a Jewish year. This will show you the correspondence between the Jewish and Gregorian dates.
There is a problem however. It seems virtually certain that during the early centuries of the first millenium C.E. the Jewish calendar was not based on a mathematical formula as it is at the present time, but was created as they went. This means that when the New Moon appeared a new month was declared, and if the spring month was approaching but there were no signs of spring, they would insert an additional month prior to the month immediately preceding the spring month, so there would be two months designated Adar, of which the second was the "real" Adar. This means that they could, if necessary, decide not to intercalate a month, even if it seemed necessary to do so, for economic reasons, because intercalating would delay the time when they could utilize the new crop according to Biblical law. Perhaps they were using a mathematical calculation to help them, but they were not obliged to follow it. For many years now, however, the mathematical calculation has ruled. The upshot of this is that we cannot be certain if the mathematically calculated date – which this calendar offers – is the date that was actually observed as such in a particular year, prior to the final acceptance of the calculated calendar around the seventh century C.E. This may be a little frustrating, but we have to face the facts!

15. Can there be a "blue moon" in the Jewish calendar?

The expression "once in a blue moon" is used to refer to a rare happening. In point of fact it is not all that rare, once every 2.7 years to be exact, because a blue moon is the second full moon occurring in the same calendar month of the civil calendar. Since the lunar month is approximately 29 ½ days long, there can never be a blue moon in February. Tom Skilling, chief meteorologist at WGN-TV in Chicago, writing in the Chicago Tribune for September 27, 2001, pointed out that there was a blue moon there on October 31, 2001 at 11.41 p.m., the first full moon having occurred on October 2 at 8.49 a.m. Curiously, this meant that the blue moon in the Eastern Time Zone occurred in the following month, November, since at that full moon the new day had already started there, and the second full moon was on November 30.
This points out that the lunar month is irrelevant to the civil calendar. New Moons and Full Moons can occur at any time during the month. Not so the Jewish calendar, where the full moon is always mid-month. It may not necessarily coincide with the astronomical full moon, because of certain adjustments that were made to the calendar for religious convenience, but there can never be two full moons in the same Jewish month.
So, if you are speaking in a Jewish context, "once in a blue moon" means – never!

[The following has not been asked frequently, but it may come up if the use of the browser spreads.]
16. I tried to run your program on the new Opera browser, but it did not seem to work properly. Please explain.

The new Opera browser effectively strips off the plus sign from any number you insert. In most browsers, a plus sign entered by the user is encoded, because the plus sign is employed by the browsers instead of the space, so as to create a spaceless string for processing. The Opera browser does not encode the plus sign, apparently because the programmers decided that an unsigned integer is always equivalent to an integer preceded by a plus sign. But it is not for me! I regard this as a bug, and have notified the owners of the browser. But if they feel it is a feature rather than a bug, the problem will continue, because I do not find it worth while to change the program.
There is a simple solution. Instead of writing, for example, +2002 write 2002CE or 2002AD, and you will then get a correct result. Do not leave a space between the numeral and the following letters. Do leave a space before another year that you enter, however.