Minor edit to above post: the routines that would require work to account for the Gregorian reformation are actually the Gregorian routines, not the Hebrew date routines.

Golang: FromRD, ToRD

https://github.com/hebcal/greg/blob/main/greg.go

ECMAScript: greg2abs, abs2greg, toFixed, yearFromFixed, etc.

https://github.com/hebcal/hebcal-es6/blob/main/src/greg0.js

Understood. And yes, you're correct that this is a bug. Any date conversion between Gregorian and Hebrew calendars involving dates prior to the year 1752 C.E. (Hebrew year 5512) or earlier is guaranteed to be inaccurate.

Hebcal does not take into account a correction of eleven days that was introduced by Pope Gregory XIII known as the Gregorian Reformation. Wednesday, 2 September 1752, was followed by Thursday, 14 September 1752.

This has been a known issue with Hebcal for 30 years, and we haven't yet found the time to fix the bug.

If you're a programmer, we'd welcome a pull request. 

You could either start with Go implementation of Hebcal (look at the FromRD/ToRD functions)

https://github.com/hebcal/hdate/blob/main/hdate.go

Or with the ECMAScript/JavaScript implementation (look at functions abs2hebrew and hebrew2abs) 

https://github.com/hebcal/hebcal-es6/blob/main/src/hdate0.js

Daniel, thanks for this. I really appreciated the background reading, so thanks for sharing those sources! I realize I misunderstood your initial question.

To answer your question about calculations, Hebcal doesn't count forward from the creation of the world. Rather, it calculates dates using a set of mathematical formulas. The dates all are relative to an "epoch", which is an arbitrary point in time. In our case, the epoch is set to the arbitrary/imaginary date 1 Tishrei 1. Once the epoch is fixed, all further calculations are done relative to the epoch.

In any case, your original question asked: Why does HebCal return -3759?

If that's still an important thread, could you send a screenshot or a URL of some place where Hebcal is displaying -3759 with respect to Hebrew Year 1? 

Hi, thanks for using Hebcal!

To get Shabbos/Yom Tov times for your city, visit our custom calendar creator at https://www.hebcal.com/hebcal

Check the various holiday/event options you want, and then type the name of your city in the "Candle-lighting & Fast times" box, and then pick from the drop-down list. See screen-shot below.

Then, click Create Calendar.

Then, you'll be able to click the Download button and select your calendar app (Outlook, Google Calendar, Apple, etc)

1st of Tishrei, Hebrew Year 1 = Mon, 7 September 3760 B.C.E.

https://www.hebcal.com/converter?hd=1&hm=Tishrei&hy=1&h2g=1

To find the corresponding Jewish year for any year on the Gregorian calendar, add 3760 to the Gregorian number, if it is before Rosh Hashanah. After Rosh Hashanah, add 3761.

You're welcome! Enjoy studying the Daf!

Hi, thanks for using Hebcal.

Can you send a screenshot or a URL? The Hebrew date conversion may be confusing to understand, and we might be able to help explain to you what results you're seeing.

We've added a feed with the Schottenstein page numbers to this page https://www.hebcal.com/ical/

Because the Vilna/Schottenstein switch will take up some space, we need to do a small form redesign to make that fit.

Are you using the Schottenstein edition or the Vilna page numbers? We can more quickly add a Schottenstein iCalendar feed to the simple download page  https://www.hebcal.com/ical/ as that doesn't require the aforementioned page redesign of the custom calendar builder.

Hi, thanks for using the Hebcal Yahrzeit + Anniversary calendar.

To view the calendar on Android, we recommend syncing your Hebcal personal yahrzeit calendar to Google Calendar, following these instructions.

https://www.hebcal.com/home/60/google-calendar-jewish-holidays

Note that you can also subscribe to annual email reminders

https://www.hebcal.com/home/4339/yahrzeit-anniversary-calendar-annual-email-reminders