Jewish Calendar Conversion Program
Frequently Asked Questions

Stephen P. Morse , San Francisco


100 Conversion Page
200 Yearly Calendar Page
300 Miscellaneous
400 Technical Difficulties with Website
See also Rules for Jewish Calendar

Section 100 -- Conversion Page

101. How do I convert a common date to a Jewish date or vice versa?

Simply use the dropdown lists to specify the common or Jewish date that you want.  That's it -- no "compute" button to press.  The answer will appear instantaneously in the other fields.

102. How do I convert a common date prior to 1939 or a Jewish date prior to 5700?

To do that you need to change the century field.  You can specify a new century either in the Jewish era (current Jewish century is 58th) or in the common era (current common century is 21st).

103. How do I find out when my child's bar/bas mitzvah date will be?

Enter your child's common date of birth.  The Jewish date of birth will instantaneously appear.  Modify the Jewish year to be 13 higher.  Then look again at the common date for your answer.  Of course you'll probably want to use the Saturday that immediately follows that date.

And if your child was born after sundown, then you'll have to add one day to the Jewish date.  See question 106 for a discussion on this.

104. What are the encoded dates?

Just as Roman numerals are an encoding that expresses common years as Latin letters, there is an encoding that expresses Jewish years as Hebrew letters.  Such an encoding is often found on Jewish tombstones.More information about the encoding can be found by pressing the "Calendar Rules" button and going to the section entitled "Date Encodings used on Tombstones".

If you are using one of the older browsers (such as Netscape 4), you will see a "Display Encoded Date" button.  You will need to press this button in order to see the encoding of the currently selected date.  The reason for this button is that it takes time to download the Hebrew letters used in the encoding, and I didn't want people who were uninterested in the encoding to have to wait while this download occurs.   The new browsers use text rather than images for the encoded dates and consequently they don't have to download anything.  So the encoded date is always displayed if you are using one of the newer browsers.

105. Why is there a missing month between Adar and Nisan in the Jewish month dropdown list?

That corresponds the the leap month of Adar-2 which occurs only in certain years.  Click on the "Calendar Rules" button for more information on the leap month.

106. Your conversion is off by one day for my grandmother's yahrtzeit date that was given to me by the funeral home.  Why is that?

If your grandmother died after sundown but before midnight, then a new Jewish day had already started but the secular date is still the old day.  My conversion program doesn't know if the event you are interested in occured after sundown or not, so you'll have to make that adjustment yourself.

By the way, I've discovered that many people don't know why the Jewish day starts at sundown.  Here's why.  According to the biblical story of creation (Genesis 1:5), first it was dark.  Then God created light.  So the darkness preceded the light.  "And it was evening and it was morning, the first day."  So the first day and every day thereafter starts with a period of darkness and ends with a period of light.

107. Why do I find Hebrew-letter encoding for the the day of the month that are different from what's shown in the Encoded Date?

The Encoded Date shows the simplest combinations--using the base-10 numbering system (e.g., 12 is shown as the letters with the values for 10 and then 2, right to left), but any combination of letters whose values add up to the intended number are equally correct--and the order of the letters is irrelevant. (For the letter values, see Date Encodings used on Tombstones in the Calendar Rules.) Thus, 12 could also be represented by the letters whose values are 9 & 3, or 8 & 4, 7 & 5, etc.

One regular deviation from the expected, simplest, combination is where the letters would spell out the name of G_d. So for 15 & 16, rather than the letters for 10 & 5 (YUD HAY) and 10 & 6 (YUD VAV), those numbers are normally written with the letters for 9 & 6 (TESS VAV) and 9 & 7 (TESS ZAYIN).

108. Do the Torah portions shown apply in Israel?

The portions shown are for the diaspora.

Torah portions between the diaspora and Israel often differ the further away we get from the beginning of the year.  That's because some holidays are two days in the diaspora but only one day in Israel.  If the second day falls on a Saturday, the holiday portions of the Torah are read in the diaspora but not in Israel.  To even it all out, towards the end of the year many more portions are doubled up in the diaspora than in Israel.

[Second thoughts: I now display the torah portion both for the diaspora and for Israel.]

109. Why does the first day of year 1 fall on a Monday?

If you plug in Tishri 1 in the year 1, you will see that it occurred on a Monday.  That seems strange if you believe that creation started on Tishri 1, in which case it would have had to be on a Sunday to make the seventh day be Saturday.

There are actually two incorrect assumptions in the preceding paragraph according to Jewish tradition.  First is that the creation started on Tishri 1.  Another interpretation is that Tishri 1 marks the completion of creation -- namely the first Sabath.  In other words creation started on the 24th of Elul and ended six days later on the 29th of Elul, the day before Tishri 1.  But that too doesn't agree with what my program shows -- my program has Tishri 1 in the year 1 being on a Monday, not a Saturday.

The second incorrect assumption is that the first year was year 1.  Actually year 1 was a fictitious year introduced to make the calculations correct.  Creation started on 24th of Elul in the year 1 (year 1 prior to 24 Elul did not exist) and the first Sabath was on Tishri 1 in the year 2.  And, indeed, my program shows that Tishri 1 in the year 2 does fall on a Saturday.

This leads to two different ways of counting years.  One is the counting method in common use, starting with the year that never existed.  That method of counting is called Aera Mundi.  The other is the counting method used in the Talmudic and Geonic* period and starts with the year that began upon the completion of creation.  That method of counting is called Aera Adama.

* The Geonic period is the period in Jewish history, approximately 700-1000 CE when the Geonim (the heads of the two Yeshivot, the Jewish centers of learning in Babylonia ) were the transmitters of traditional Judaism.

[Second thoughts]

After writing the above, I received an e-mail with the following very interesting (and mathematically correct) interpretation of the exact start of creation.

It is not correct to say that Creation began on the 24th of Elul.  The Talmud and every other authority I've ever come across unanimously agrees that Creation week began on 25th Elul (which should be a Sunday).  Moreover, Adam and Eve are believed to have been created on Friday, 1st Tishri, so that G-d could rest on Saturday, Shabbat (the 2nd of Tishri).  Your program lists these dates as occuring one weekday later in the week than they should, because your program invokes the "Sunday-Wednesday-Friday rule" to delay the 1st of Tishri by one day in Year 2.  Yet, there's no reason to believe that the "Sunday-Wednesday-Friday rule" was in effect in Year 2, and there'd be no need for it in any case, because Adam and Eve had not yet sinned by the start of 1 Tishri, and would've had no predetermined need to fast on Yom Kippur at the time of their creation.

In sum, since it's not reasonable to assume that G-d intended for Adam and Eve to fast on that first Yom Kippur, there would've have been no reason for Him to delay 1 Tishri to fall on a Saturday.  Thus, 1 Tishri must have fallen on a Friday in Year 2 (with Creation week beginning on Sunday, 25 Elul).  I believe if you can correct your program to not delay 1 Tishri in Year 2, your program would be consistent with all (known) Talmudic and rabbinical accounts.

 I'm not a Talmud scholar (far from it) so I can't comment on the Talmudic interpretation.  However I do agree that what is said above is mathematically consistent -- if you accept that there was no reason to delay the start of year 2, then creation would have started on 25th (not the 24th) of Elul in the year 1.  Therefore I have modified my program as suggested above so that Tishri 1 in the year 2 falls on a Friday and not a Saturday.

I would like to acknowledge Noam Kaplan who first taught me about Aera Mundi and Adama, and about year 1 being a fictitious year.  And my thanks to Dean Solomon for enlightening me further about creation starting on the 25th rather than the 24th of Elul, as explained under "second thoughts".

Section 200 -- Yearly Calendar Page

201. Why are certain dates on the calendar hilighted?

These correspond to the Jewish holidays.  Look at the list of holidays to the left of the calendar to see which date corresponds to which holiday.

202. Why are certain dates on the calendar underlined?

These correspond to yizkor and/or yahrtzeit dates.  Look at the left of the calendar for a list of these dates.  The yizkor dates are the same for everybody but the yahrtzeit dates are specific to each family.  They correspond to the dates of death for family members.  See question 203 for instructions on adding your family yahrtzeit dates.

203. How do I add a yahrtzeit date for a member of my family?

Go to the "Family Yahrtzeit Dates" section at the bottom left.  Enter the person's name and the Jewish month and day of death.  Then press the "Add" button.

If you don't know the Jewish date of death, you'll have to go the converter page and enter the common date of death.  The Jewish date will immediately appear.

204. Do I have to reenter my family yahrtzeit dates each time I return to your webpage?

No.  The information you entered is saved in a cookie on your hard disk.  So as long as you are using the same machine, you don't have to ever reenter your family yahrtzeit dates again.

205. How do I print out the entire yearly calendar?

This page contains several frames, and you would want to print only the frame that contains the calendar itself.  So you need to first select the calendar frame.  You do this by placing the mouse anywhere in that frame and pressing the left mouse button.

Next you go to the file menu.  In the Netscape 4.x browser you select the item that says "Print Frame".  In the Netscape 6/7 browsers and the Microsoft browsers you select the item that says "Print" and make sure that the radio button saying "Only the current frame" (Microsoft) or "The selected frame" (Netscape)  is checked.

Second Thoughts:

I've now implemented an easier way to print out the calendar.  One the bottom of the yearly calendar page click on the button that says "Printer Friendly Version" and that displays only the frame that contains the calendar.  Now you can print out the calendar alone by selecting the normal Print command from the File menu.

But before you start printing, you want to make sure that the background colors get printed as well.  Otherwise there will be no visual separation between adjacent months.  The way you specify that you want background colors printed depends on the browser you are using:

Microsoft: From the Tools Menu select Internet Options.  Click on the Advanced tab.  Under Printing check off "Print background colors and images"

Netscape 6/7: From the File menu select Page Setup.  Check off "Print Background (colors & images)"

Netscape 4: Background colors are always printed

Section 300 -- Miscellaneous

301. What do negative numbers in the common year mean?

Remember that there was no common year 0.  The year preceding the common year 1 is the year -1 on my dropdown list.  So this would correspond exactly to what is meant by 1 BCE (before the common era).  Similarly -100 would be 100 BCE, etc.

[Second thoughts: Negative year numbers are no longer used. Common years are now displayed explicitly as CE or BCE.]

302. Are you displaying Gregorian or Julian dates?

The Julian calendar for commmon dates predated our current Gregorian one.  In the Julian calendar, every year that was divisble by 4 was a leap year.  This caused the seasons to drift.  So the Gregorian calendar was introduced to rectify that.  It did so by requiring that a year that is divisible by 100 not be a leap year unless it is also divisible by 400.  When the switch from the Julian to the Gregorian calendar occured, ten days were suddenly skipped so as to get the seasons back in the right place.  The switch was ordered by papal decree in 1582, although it was not widely adopted until the 18th century and later in some places (e.g., Alaska in the 19th century, Russia in the 20th century).

All common-era dates shown on my website are Gregorian dates.  In other words, the dates that I show for early years are calculated by backing up, assuming that the calendar rules we have today were always in effect.

[Second thoughts: I've now added a radio button so you can select whether you want the common-era dates to be in Gregorian or Julian.  The default is Gregorian.]

303. How would I express Jewish dates as numbers (e.g., 5766-9-27) since different years have different numbers of months?

Your initial impulse would be to number the months with the Tishri (the first month) being 1, Heshvan (the second month) being 2, etc.  But then when you get to Nisan you will have a problem because in leap years there is an extra month preceding Nisan; it will be the seventh month in non leap years and the eighth month in leap years.

To solve this problem, start numbering the months from Nisan.  In other words,Nisan is month 1, Iyyar is month 2, etc.  That may seem odd, but it is consistent with the religious view of Nisan being the first month (Exodus 12:2, This month [of Nisan] shall be to you the beginning of months; it shall be the first in the months of the year).  So even though the year number changes when you get to Tishri, the month number does not cycle back to 1 until you get to Nisan.  And that completely solves the leap-year problem.

Section 400 -- Technical Difficulties with Website

401. Why don't I see the buttons (Display Yearly Calendar, Show Year Properties) at the bottom of the page?

You probably have your screen resolution set too low.  Go to the control panel and increase the resolution to at least 800x600 (higher is even better).

402. Why doesn't the encoded date (hebrew characters) redisplay properly when I change the date?

That problem was reported on a mac running the IE 5.1 browser.  I'm awaiting further information to determine if this is occuring with IE 5.1 on a pc as well.

Until I figure out what the problem is, one work-around is to click the hide/redisplay button twice after each time that you change the date.  Or you could switch to a mac netscape browser -- that one was been reported not to have this problem.

403. Why do I get an error message for the (non-existent) page when I use this calendar program?

This one I investigated and discovered it too was a problem with mac only, specifically with the Safari browser.  Here is what I learned.

My code has a line in it that reads"text/html","replace");

That is supposed to open a frame and insert into it a page of type "text/html".  Now there is another kind of open command that has the form, ...)

and in that case the first parameter is the url of the page that is supposed to be opened.  For some reason the Safari browser is treating the command as if it were a command and in that case it is looking for the page at url "text/html".  Since you were at when you executed the command, the specific page it tries to open is  Hence the error message.

This is a very serious problem in the Safari browser and I'm surprised that more websites aren't failing because of it.  The suggested work-around in this case would be to use the IE browser instead.

But when I suggested that, I was informed of another problem on the mac.  Specifically IE on the mac does not have support for Hebrew characters, which is why Mac users are running Safari instead.  What can I say.  It looks like mac user are in a catch-22 here.  You either use Safari for Hebrew characters in which case the code that is supposed to open up new windows doesn't work.  Or you use IE so that windows can open properly in which case you don't get Hebrew fonts.  My advice in this case is still to use IE to view my calendar website since the hebrew fonts are only used in one place.  That one place is on the first page before my program attempts to open other windows, so if you want to see the hebrew font there, use Safari just to view the first page.

Not a completely satisfying solution but is the best that I can offer mac users.  The problem is not mine but rather two serious bugs, one in each of the two popular mac browsers.  I wonder what would happen if you tried the Netscape 7 browser on the mac, and whether it would have either of those problems.

[Second thoughts:  Another user has reported to me that the Mozilla browser on the Mac doesn't have either of these problems with my Jewish Calendar page.  Since the Firefox browser and the Netscape 6/7 browsers are all based on the same underlying source code as the Mozilla browser, they obviously won't have the problem either.  So my suggestion is to use one of these browsers on the Mac instead of Safari or IE if you want to view my Jewish Calendar page.]

[Third thoughts: Another user has reported to me that the latest version of Safari no longer has the problem.  I do not have a mac so I am unable to verify this.]

-- Steve Morse