Date Conversion Utility
Provided below is a "two-way" utility. You can choose between a Gregorian-to-Jewish date conversion or a Jewish-to-Gregorian one. When supplying the date to be converted, the time of the year can be selected either by day-and-month combination or by holiday (or other significant day of observance).
While determination of the dating sequence for the Jewish calendar (as well as the weekly cycle) is pretty straightforward (from Monday, the first of Tishri of year 1 on), that of the Gregorian calendar can be somewhat confusing. Based upon a number of resources that I have consulted, it appears that the Gregorian calendar was not truly implemented until around 1582. Even then, not all parts of the Western world immediately accepted this new dating system. Many more places would "go Gregorian" several years later. I have gotten the impression that a Roman dating system, referred to as the Julian calendar, was utilized until the Gregorian switchover. It appears that the Julian calendar was put into effect back around the time of the Roman empire (a period that included 1 CE). I have also read that every 4th year was a leap year, even those years ending in "00", whether they were evenly divisible by 400 or not. This would amount to a simple difference between the Julian and Gregorian calendars (centennial years not divisible by 400, being stripped of leap year status under the Gregorian system, while keeping this status under the Julian system).
A really tricky part is to picture any of these two calendars as if it had been in effect before the Roman empire. How correct would such a dating system be back then? In light of all this confusion, I have decided, at least for now, to use a type of Gregorian system stretching all the way back to the very first Rosh Hashanah. It's not perfect—most of the time this Jewish holiday occurs in August during the most ancient years—but this is what I am going with for now. Keep in mind, however, that the Jewish and Gregorian dates will still be properly matched by the conversion utility as long as the date involved is October 15, 1582 CE or later. Thus this utility will certainly come in handy for more recent, modern times. But any earlier date is subject to considerable dispute. I have read that Pope Gregory (after whom the Gregorian calendar was named) eliminated a period of ten days in 1582 CE, from October 5 to 14 inclusively. This was to compensate, likely, for the Julian calendar (supposedly named after Roman emperor Julius Caesar) containing too many days, i.e., having all the 400-resistant centennial years retain their February 29's—that's 12 extra days between 1 and 1582 CE! Applying this centennial leap status elimination/inclusion to the BCE period alone, from the first Rosh Hashanah (whether 3761 or 3760 BCE) to 1 BCE, results in a difference of 28 days. So which calendar should be used, and when? It appears that the Gregorian calendar is, with its 1582 adjustment, a continuation of the Julian calendar going back to 1 CE (allowing all the centennial years between 1 and 1582 to keep their leap days). But which calendar should be used before then? Perhaps the Gregorian with its centennial leap eliminations? I have tested this approach using a study Bible, but could not get the dates to line up to my liking.
Until I get more information to my satisfaction, this is what I have decided—out of simplicity—to do with my utility, in regard to dates before October 15, 1582 CE (AD):
1) Pope Gregory's 10-day adjustment will not be included (this is subject to change in the future). The utility will therefore accept all dates in October 1582, including the 5th through the 14th.
2) February 29 will be eliminated for all centennial years not evenly divisible by 400. This includes those between 1 and 1582 CE, despite the Julian calendar keeping them intact (but while 12 days are hereby lost in this particular period, omission of the 1582 adjustment reduces this loss to only 2 days).
3) 1 CE immediately follows 1 BCE (no intervening year 0). Therefore a BCE year's leap status is determined by subtracting 1 from its number and applying the result to the same tests as the number of a CE year (e.g., BCE years like 1, 5 and 401 are leap, but BCE years like 4, 101, and 400 are not).
Perhaps this method could be referred to as one that involves a purely non-stop Gregorian pattern without any special one-time adjustments (such as that of 1582). But I'd like to get a sufficient amount more info before I put my efforts into writing special code to handle the Julian calendar for select periods of time. Therefore I am, for now, using the approach that eliminates the 10-day adjustment of 1582 and all the 400-resistant centennial leap days. Under this arrangement, Rosh Hashanah of the Jewish year 1 coincides with September 7, 3761 BCE. (Inclusion of the 1582 CE adjustment would place this holiday at August 28, 3761 BCE. An intervening year 0 between 1 BCE and 1 CE would place this first Rosh Hashanah in 3760 BCE.)
So go ahead and use the conversion utility below. Remember that if the date to be converted involves October 15, 1582 CE or later, you can expect a proper match, with (hopefully) no dispute (I am guessing that most inquiries will be for these modern times anyway). But any earlier date is disputable. I must add this disclaimer: Although I have extensively tested this utility in order to ensure its accuracy, I do not accept any liability for any errors or damages that may result from usage, of this or any other utility, or of anything else on my web site (hey, end-user license agreements that accompany so many programs are like this as well; those program providers don't want to jeopardize themselves—I don't blame them—and neither do I).
Enter an unsigned (positive) integer for the year (up to 4 digits). If you choose to input a Gregorian date, remember to select either the Common Era or Before Common Era entry as well (e.g., if a Gregorian year before the Common Era is to be used, simply enter the number of that year, and select the Before Common Era entry). Remember that this utility uses the system that places the year 1 CE immediately after 1 BCE (no intervening "year 0", so do not enter 0 for the year). This means that while Rosh Hashanah for the Jewish year 3762 took place during the Gregorian year 1 CE, Rosh Hashanah for the Jewish year 3761 (not 3760) took place during the Gregorian year 1 BCE. Thus valid Gregorian BCE years are 1 through 3761.
Substitution Checkbox for Jewish-to-Gregorian Conversions
When this box is checked, the year entered in the Jewish-to-Gregorian section is treated as Gregorian (note: only Common Era, i.e., "AD" years are accepted) rather than Jewish. In such a case, this convenient feature (great for determining Jewish holidays within a Gregorian calendar!) automatically selects a corresponding Jewish year, based upon the Gregorian year and the time of the Jewish year entered.
For a given Gregorian year, there are two possible Jewish years that overlap it. One of these Jewish years ends up being utilized. Which one is chosen depends on the time of the Jewish year entered.
If that time is Tevet or an earlier month, then the later Jewish year matching the entered Gregorian one is used. Note that this includes Rosh Hashanah (i.e., the day itself—the 1st day of Tishri, not the immediately preceding Erev!), Chanukah (25th day of Kislev) and all other holidays in between. Please keep in mind that Erev Rosh Hashanah is not included here.
However, for months later than Tevet, the earlier matching Jewish year is used. Note that this includes Tu b'Shevat (15th day of Shevat) and Tisha b'Av (9th day of Av) and all the holidays in between them. Furthermore, Erev Rosh Hashanah is also included, because it is treated as the 29th day of Elul, the last day of the Jewish year (not the 1st day of Tishri!).
It is important to also note that in some cases, the resulting Gregorian year after the Jewish-to-Gregorian conversion may not match the one entered as a substitute for the Jewish year here, but rather be adjacent to it. The closer to January 1st the Gregorian result is, the more likely this is going to happen. Hopefully, this should practically never be a problem when selecting a Jewish holiday other than Chanukah. However, even if a Chanukah selection results in a differing Gregorian year, that may be desirable anyway, due to this holiday running extremely late with respect to the Gregorian calendar. In other words, the Chanukah normally expected around December in a given Gregorian year (entered as a substitute for a Jewish year) may, in fact, take place in January of the following one, which would show up as the correct year in the Gregorian results.
Like the Jewish year evaluation utility (in Section 9), the above date converter uses CGI (Common Gateway Interface).
This wraps up my Jewish Calendar discussion at this point. I hope to have an informative yearly holiday profiler sometime in the future. For now, enjoy what I have here. All this has been in the making (off and on) since at least early 1999 CE (AD).
In closing, I would like to invite you to check out a project that was more recently done, around 2012, by a fan of these Jewish Calendar pages. Nathaniel Sherrill has utilized my pages while putting together a book. The result of this work is The Layman's Gospel Harmony, from Move Mountain Press, featuring an effort in presenting the four Gospels of the New Testament in a linear timing arrangement.