The modern definition is when the center of the sun is 7° below the geometric (airless) horizon, somewhat later than civil twilight at 6°.The beginning of the daytime portion of each day is determined both by dawn and sunrise.Even with this intercalation, the average Hebrew calendar year is longer by about 6 minutes and 40 seconds than the current mean tropical year, so that every 216 years the Hebrew calendar will fall a day behind the current mean tropical year; and about every 231 years it will fall a day behind the mean Gregorian calendar year., "from the creation of the world"). AM 5777 began at sunset on 2 October 2016 and will end at sunset on 20 September 2017.
Maimonides' work also replaced counting "years since the destruction of the Temple" with the modern creation-era Anno Mundi.
The Hebrew lunar year is about eleven days shorter than the solar year and uses the 19-year Metonic cycle to bring it into line with the solar year, with the addition of an intercalary month every two or three years, for a total of seven times per 19 years. or AD), the words or abbreviation for Anno Mundi (A. or AM) for the era should properly precede the date rather than follow it.
In Judaism, an hour is defined as 1/12 of the time from sunrise to sunset, so, during the winter, an hour can be much less than 60 minutes, and during the summer, it can be much more than 60 minutes.
This proportional hour is known as a 'sha'ah z'manit' (lit. Instead of the international date line convention, there are varying opinions as to where the day changes. (Jerusalem is 35°13’ east of the prime meridian, so the antimeridian is at 144°47' W, passing through eastern Alaska.) Other opinions exist as well.
Within the Mishnah, however, the numbering of the hours starts with the "first" hour after the start of the day.