the solar day decreed by God and the day of men?

And God called the light, Day. And He called the darkness, Night. And it was evening, and it was morning, day one. (Genesis 1:5 KJ3) 

In the normal revolution of the sun, when the sun suddenly disappears into our horizon, and at the same time rises upon others , darkness comes to us, it is night. With darkness of the night, we greet each other by saying 'good evening'. At the end of the conversation, we do not say again 'good evening' but 'good night' to include the remainder of the darkness of the night until the next appearance of the sun, when we greet each other 'good morning". After our local "twelve" o'clock, we say 'good afternoon' until darkness returns to mark night. That is trivial, because everybody knows when to say good morning, good afternoon, good evening, good night. Any of those divisions of the day can be chosen to begin the day, according to what mankind would agree upon in the future. For example, when I wrote this, where I live, in the State of Florida of the United States of America as in any other geographical location that keeps time under the Gregorian Calendar, a day legally starts a second after twelve o'clock midnight, not when the day is returning again. A change in the reckoning of time can be detected in the naming of our English months September (seventh), October (eight), November (ninth), December (tenth). In the Gregorian calendar those months occupy different spots: December is the twelfth month not the tenth month at it was at one point in time. Not all the calendars shifted. The calendar of Moses remains, even though those to whom it was entrusted also chose a different calendar to carry out their business.

[God was not interested in the exact atomic time keeping when he created our time. The revolution of the sun marks the day length according to our senses.]

So, to understand the Scriptures, we cannot understand the time and calendar day mentioned in the Scriptures as our modern time and day, because the day in the Scriptures had been fixed to a starting point which is the evening, as it is written:

And God called the light, Day. And He called the darkness, Night. And it was evening, and it was morning, day one. (Genesis 1:5 KJ3)

The idea of evening in the Scriptures has lost itself in our daily conversations and in division of time. For example, we begin the observance of the Sabbath day when darkness begins, even though we read that  the phrase 'there was an evening and there was a morning' constitutes a day. If we begin the Sabbath when darkness begins it is because we take evening to be the same as night which is the name for the darkness (Genesis 1:5). Even if the chosen people would decide to start their Sabbath day and divide the day as they see fit, we must seek guidance from the spirit of the Law of Moses (Luke 26:29) which is eternal (Matthew 5:17).

What is the evening?

Luke 16:29 EMTV

(29)  Abraham said to him, 'They have Moses and the prophets; let them hear them.'

 It took some time for the demarcation of evening from night to be revealed as I was searching for it in the Bible. It is obvious when evening ends, that is, when darkness settles in. But, when does evening begin?

I look at  some events in the Bible that occurred in the evening time to conclude that evening could not have been a time after daylight ends and darkness (night) begins.

The best unambiguous example of evening time being within the daylight before evening is read in the narrative of the envoy of Abraham staying by the well of water waiting for the ladies to come draw water. That time of evening could not have been at  or after sunset:

Genesis 24:11 KJV

(11)  And he made his camels to kneel down without the city by a well of water at the time of the evening, even the time that women go out to draw water.

In the book of Joshua, we read that the body of the slain king of Ai was taken down when evening came and at the going of the sun was handled. The actions carried out indicate that evening could not have after or at sunset, as the phrase reads 'at the going of the sun'.

Joshua 8:29 Darby

(29)  And the king of Ai he hanged on a tree until the evening; and at the going down of the sun Joshua commanded, and they took his carcase down from the tree, and threw it down at the entrance of the gate of the city, and raised upon it a great heap of stones, which remains to this day.

We have a similar evening event scenario in the evening soliciting of the body of Jesus and His burial that indicates that evening could not have been at sunset.

Mark 15:42-44 EMTV

(42)  And now evening having come, since it was Preparation Day, which is the day before the Sabbath,

(43)  Joseph of Arimathea, a prominent council member, who was himself waiting for the kingdom of God, coming and taking courage, he went in to Pilate and asked for the body of Jesus.

(44)  And Pilate wondered if He was already dead; and having summoned the centurion, he questioned him if He had been dead long.

Sunset ends the time of the evening. 

When does evening or the Mosaic day begin? 4 AM.

The day, by default, lasts 12 hours but we know that the length of the day does vary. That division was asserted by Jesus close to the time of Passover.

John 11:9 EMTV

(9)  Jesus answered, "Are there not twelve hours in the day? If anyone walks in the day, he does not stumble, because he sees the light of this world.

In the parable of  Matthew 20, the time slots, the 4 watches, are clearly delineated: early morning, 3rd hour, sixth hour, ninth hour, the twelve hour because the the last hired person worked only 1 hour before everybody got paid. We see a division of 4 watches of 3 hours each in the parable. The division must be the same for the watches of the night which can deduce from the watches of the day: the evening watch, the third hour, the sixth hour, the ninth hour. 

Matthew 20:1-12 EMTV

(1)  "For the kingdom of heaven is like a certain landowner who went out early in the morning to hire workers for his vineyard.

(2)  And having agreed with the workers for a denarius a day, he sent them into his vineyard.

(3)  And going out about the third hour, he saw others standing idle in the marketplace.

(4)  And said to them, 'You also go into the vineyard, and whatever may be right I will give you.' So they went.

(5)  Again going out about the sixth and the ninth hour, he did likewise.

(6)  And about the eleventh hour, going out he found others standing idle, and said to them, 'Why do you stand here idle all day?'

(7)  "They said to him, 'Because no one has hired us.' He said to them, 'You also go into the vineyard, and whatever is right you shall receive.'

(8)  "So when evening came, the owner of the vineyard said to his foreman, 'Call the workers and pay them their wages, beginning with the last to the first.'

(9)  "And when those came who were hired about the eleventh hour, they each received a denarius.

(10)  But when the first came, they supposed that they would receive more; and they also received each a denarius.

(11)  And when they had received it, they began grumbling against the landowner,

(12)  saying, 'These last men have worked only one hour, and you made them equal to us who have borne the burden and the heat of the day.'

The Jews sought reprieve from Caesar, according to Josephus, not to be disturbed in the time of preparation which was after the 9th hour:

• "Caesar Augustus, high priest and tribune of the people, ordains thus: Since the nation of the Jews hath been found grateful to the Roman people, not only at this time, but in time past also, and chiefly Hyrcanus the high priest, under my father (7) Caesar the emperor, it seemed good to me and my counselors, according to the sentence and oath of the people of Rome, that the Jews have liberty to make use of their own customs, according to the law of their forefathers, as they made use of them under Hyrcanus the high priest of the Almighty God; and that their sacred money be not touched, but be sent to Jerusalem, and that it be committed to the care of the receivers at Jerusalem; and that they be not obliged to go before any judge on the sabbath day, nor on the day of the preparation to it, after the ninth hour. But if any one be caught stealing their holy books, or their sacred money, whether it be out of the synagogue or public school, he shall be deemed a sacrilegious person, and his goods shall be brought into the public treasury of the Romans. And I give order that the testimonial which they have given me, on account of my regard to that piety which I exercise toward all mankind, and out of regard to Caius Marcus Censorinus, together with the present decree, be proposed in that most eminent place which hath been consecrated to me by the community of Asia at Ancyra. And if any one transgress any part of what is above decreed, he shall be severely punished." This was inscribed upon a pillar in the temple of Caesar. Antiquities 16.6.2 

Incidentally, Jesus died at the ninth hour (Matthew 27:46-51)

Matthew 27:45-50 EMTV

(45)  Now from the sixth hour until the ninth hour, darkness came upon all the land.

(46)  And about the ninth hour Jesus cried out with a loud voice, saying, "Eli, Eli, lima sabachthani?" that is, "My God, My God, why have You forsaken Me?"

(47)  Some of those standing by, when they heard this, said, "This Man is calling Elijah!"

(48)  Immediately one of them ran and took a sponge, and filled it with sour wine and put it around a reed, and offered it to Him to drink.

(49)  But the rest said, "Leave Him alone; let us see if Elijah is coming to save Him."

(50)  But Jesus, again crying out with a loud voice, released His spirit.

The Gospel of John speaks of the appearance of Jesus in front of Pilate  which he places around the 6th hour on the Preparation of the Passover:

 John 19:14 (Latin Vulgate Gallican 405 AD)

(14)  erat autem parasceve paschae hora quasi sexta et dicit Iudaeis ecce rex vester

You do not know latin but see the word "sixth" in "sexta". That Latin quote is to establish the antiquity of the sources placing the event at the 6th hour from John's narrative. 

John 19:14 RV

(14)  Now it was the Preparation of the passover: it was about the sixth hour. And he saith unto the Jews, Behold, your King!

John was an eyewitness of the crucifixion. He stood right there at the foot of the crucifixion (John 19:25-27). He could not have mistaken the time.

John 19:25-27 RV

(25)  But there were standing by the cross of Jesus his mother, and his mother's sister, Mary the wife of Clopas, and Mary Magdalene.

(26)  When Jesus therefore saw his mother, and the disciple standing by, whom he loved, he saith unto his mother, Woman, behold, thy son!

(27)  Then saith he to the disciple, Behold, thy mother! And from that hour the disciple took her unto his own home.

(Also, Jesus made the declaration that a day contains 12 hours in John 11:9).

Jesus told the parable right before Passover, in April, first or second week, when the day is longer than 12 hours in Jerusalem. 

While John reports the time of the sentencing to death and the crucifixion at about the 6th hour, the Gospel of Mark places the crucifixion at about the 3rd hour.

Mark 15:25 RV

(25)  And it was the third hour, and they crucified him.

Rather than correcting the timing of John to coincide with the timing of Mark, some have correctly argued that the time of John is from a Roman perspective  while the time of Mark is the Jewish time.

The Greek words "héktos" (sixth) from John and "trítos" (third) from Mark are not that close in orthography to be confounded for each other.

Alors, from that synchronization provided by Mark 15:25 and John 19:14, a table can tabulate the equivalent time between our time (Roman, Gregorian) and the Jewish time to establish the clear demarcation of time in the time of Jesus. In 27 AD, 14 Nisan, the sun rose at about 5:21 AM and set at about 6 PM. The day lasted 12 hours 41 minutes. In 30 AD, April 5, the sun rose at about 5:26 AM and sets by 6 PM. The day lasted 12 hours and 33 minutes (7000 years calendar, June Calends).