I have noticed that the footnotes in some Bibles date biblical events to exact dates in our calendar, such as January 11, 678 BC, but the dating formulas in the EHV notes are generally more vague, such as January/February 678 BC. Why don’t you use the more precise dates?
None of the systems for translating Old Testament events into exact dates in our calendar are valid because the ancient calendars were based on subjective observation not on mathematical calculation.
The problem starts from the fact that Jewish calendar years do not match up evenly with our solar years of 365 (or 366) days, which begin in January.
The historical books of Kings and Chronicles and the prophetic books dated kings’ reigns in terms of two different calendar years: one of which began with the month of Nisan at the time of the spring equinox and the other which began with Tishri near the autumn equinox. These years did not have a uniform number of months or days. Because twelve lunar months, which total approximately 354 days, do not equal one solar year, the Jewish lunar calendars had to occasionally add a thirteenth month to a year in order to bring the lunar calendar back into line with the solar year and to keep the agricultural seasons in the right months. The result is that a calendar year with 13 months could have as many as 383 days. This problem perhaps could be overcome if the insertion of 13th months had been done on the basis of a regular mathematical rule, but the beginning of both their months and their years were not determined by a set mathematical formula but by observation, so their months and years did not follow a set mathematical pattern as our modern calendar does.
On the evening of the 29th day of a month, an observer looked for the new moon. If he saw it, the day that was beginning on that evening was the first day of a new month, and the old month was a month of 29 days. If he did not see the new moon (maybe it was there but he did not see it because it was cloudy), the new month started at the end of the next day, and the old month had 30 days. (In the traditional Muslim way of setting the start of the fast of Ramadan, the worshippers do not know what day Ramadan will start, until the actual day arrives and the observer makes his ruling.)
In ancient times, the decision to add an extra month of Adar (called Adar I) before the real Adar (which became Adar II), was based on observation: the Sanhedrin (or whoever was the authority at that point of history) observed the conditions of the weather, the crops, and the livestock, and if these were not sufficiently advanced for it to be considered “spring,” then the Sanhedrin inserted an additional month into the calendar to make sure that Passover would occur in the spring. Such a year of thirteen months was called a “pregnant year.”
Because of the irregular method of this inserting days and months into the calendar, we do not know which years in ancient times had twelve months and which had thirteen. For this reason, all systems of trying to link days of the month in these biblical books to an exact day in our calendar by mathematical calculation are not valid. Mathematical calculations perhaps can determine when that date should have occurred on our calendar. They cannot determine when it actually did occur.
In a synagogue, a young worshipper asked one of the elders, “What date is Chanukkah this year?” To which the elder replied with a smile, “It is, of course, the same date that it is every year—the 25th of Kislev.” The dates of the Jewish holidays are always the same in the Jewish calendar, but they bounce around when translated to our calendar. In modern times the correlations can be calculated mathematically. In ancient times they could not.
Another complication of calendar correlation is that throughout history, calendar systems have had occasional corrections and adjustments, and we are not necessarily aware of all of these adjustments. This problem of calendar correction is not just an ancient phenomenon. George Washington was born in Virginia on February 11, 1731, according to the Julian calendar in use at the time of his birth. So if he had a birth certificate, February 11, 1731 would be the date listed on it. However, in 1752, Britain and all its colonies switched to the Gregorian calendar, a change which moved Washington’s birthday a year and eleven days to February 22, 1732. Another recent problem caused by calendar shift is the question whether the Russian Revolution of 1917 happened in October (Julian calendar) or November (Gregorian calendar). Adding to the calendar confusion is the fact that to this day Western Christians, Eastern Christians, Jews, and Muslims each follow their own religious calendars, plus the Gregorian calendar. To complicate things further, the Gregorian, Jewish, and Islamic calendars number years from different starting points; for example, Gregorian 2018 is the Jewish year 5778, and the Islamic year 1439.
For more information on calendar complications see the appendices on chronology and the note at the beginning of Ezekiel in the EHV study Bible.