(Much of what is conveyed below are summarized notes I had used in a presentation of these principles to our home fellowship. The reference matter can be viewed in its entirety here.)
The duration of a day in the Bible is from sunset to sunset. According to the Scriptures, days begin and end at sunset and are divided into two parts, evening and morning. Thus in Genesis chapter 1 we read how “the evening and the morning were the first day,” “the evening and the morning were the second day,” “the evening and the morning were the third day” and so on.
The 24 hour biblical day is also broken into eight equal parts, each part containing three “clock hours”:
“Watches” are the description of the divisions of the night-time hours:
First watch . . . Sunset to 9 P.M.
Second watch . . . 9 P.M. to midnight.
Third watch . . . Midnight to 3 A.M.
Fourth watch . . . 3 A.M. to sunrise.
Matt 14:25 And in the fourth watch of the night Yeshua went unto them, walking on the sea.
Mark 6:48 And he saw them toiling in rowing; for the wind was contrary unto them: and about the fourth watch of the night he cometh unto them, walking upon the sea, and would have passed by them.
Luke 12:38 And if he shall come in the second watch, or come in the third watch, and find them so, blessed are those servants.
“Hours” are the description of the divisions of the daytime hours. “Hours” in the Bible typically refer to a three-hour chunk of “clock-time” during the day, but can also be describing a very specific individual clock-hour (such as “the eleventh hour,” i.e., the specific clock-hour prior to sunset).
First hour . . . Sunrise to 9 A.M.
Third hour . . . 9 A.M. to noon.
Sixth hour . . . Noon to 3 P.M.
Ninth hour . . . 3 P.M. to sunset.
Matt 20:3 And he went out about the third hour, and saw others standing idle in the marketplace,
Luke 23:44 And it was about the sixth hour, and there was a darkness over all the earth until the ninth hour.
Matt 27:46 And about the ninth hour Yeshua cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me?
Matt 20:9 And when they came that were hired about the eleventh hour, they received every man a penny.
While these divisions are apparent in Scripture, we must remember that many of the designations of time are approximate, as timepieces would not have been generally carried around by those involved in the events of the Bible. Hence, most scriptural references to hours will say “about the fourth watch,” or “about the third hour,” etc.
Additionally, the time between when last rays of the sun disappear and complete darkness sets in is called the twilight, a time of day when the breeze prevails (as when the sun sets), which is what the word means.
Prov. 7:8-9, “Passing through the street near her corner; and he went the way to her house, In the twilight, in the evening, in the black and dark night.”
With all of these divisions, however, we must remember the simplest aspect of God’s calendar is determining the divisions of the days by the setting of the sun. What easier way to know exactly when one day ends and the next begins? It is not in the middle of the night when no one can discern it without a timepiece (as per our current timekeeping method), nor is it the rising sun (as many people might still be asleep and miss it). No, YHVH in his wisdom has designated the setting sun as the beginning of the new day, when almost all people are finishing their labors of the day and preparing to rest.