At what time must we start the Sabbath Day? 4 PM EST, 5 PM DST based on John 19:14! See tabulation below.

If a biblical day starts from 'an evening to a morning', starting the Sabbath day at sunset is wrong because a day is not from 'sunset to a sunset'.

There are 12 hours in a day, says Jesus (John 11:9). We may not increase or decrease the division of the day or of the night to accommodate our geographic location. Clock time is fixed.

Jesus answered, Are there not twelve hours in the day? If a man walk in the day, he stumbleth not, because he seeth the light of this world. John 11:9 ASV

At what time does the Sabbath day start?

In Israel, the rest (sabbath) is from evening to evening that is from 4 PM, according to the tabulation of the 6th hour of John 19:14 and the 3rd hour of the other Gospels (Mark 15:25). It is important to understand the concept of evening to differentiate it from sunset or sundown, and night.

Remember that there is the definition of a day in the book of Genesis. That definition will never change. The definition states that the day starts in the evening, according to the expression 'there was evening and there was morning' (Genesis 1:4). Genesis 1:4 does NOT say that 'there was sunset and there was morning', that 'there was sundown and there was morning', that 'there was night and there was morning'.

So, if you start your biblical day, hence, your sabbath, at sunset or sundown or beginning of night or night, you are not following the standard of day that Genesis 1:4 established. Deuteronomy 23:11 clarifies evening from the other divisions in the day, placing evening before sundown or sunset, hence, before night fall.

In Antiquity, the Jews prepared for the arrival of Sabbath. At what time did they start?

In ancient Israel, Josephus left us testimony on the time when the Jews started preparing for the Sabbath. Here is a passage from Josephus, a quote that is dated around 8 BC, as referenced by the name of the Consul.

"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 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."
(Retrieved from http://www.ccel.org/ccel/josephus/complete.ii.xvii.vi.html)

Notice the time when the Jews started preparing for the Sabbath. It was after the 9th hour, Jewish time.
The 9th hour cannot be 9 AM our time, Roman time, because the Bible does not make a day start at midnight, which is the time to start to get to 9 AM. The 9th hour cannot be 9 PM either, obviously.

The Jewish day is divided into 12-hour day (John 11:9). At the 9th hour, the day is ending in 3 hours (9th-10th, 10th-11th, 11th-12th). Therefore, per Josephus, the Jews took 3 hours to prepare for arrival of the Sabbath, at the first evening daylight hour when the new biblical day starts. I have calculated the Roman time, our time, when the day starts during the Passover time in Jerusalem, based on the Jewish time mentioned in the Gospels.

When we read in the New Testament about the 'preparation day', we know that it started after the ninth hour, Jerusalem time, from the testimony of the Historian Josephus. Jewish time and Roman time are two different times, as we can infer from Mark 15:25 and John 19:14, because the Jewish day starts in the evening with daylight, unlike Roman time.

Mark 15:25 LITV
And it was the third hour, and they crucified Him.

Mark 15:33 LITV
And it being the sixth hour, darkness came over all the land until the ninth hour.

John 19:14 LITV
And it was the Preparation of the Passover, and about the sixth hour. And he said to the Jews, Behold, your King!

Mark 15:34 LITV
And at the ninth hour Jesus cried with a loud voice, saying, Eloi, Eloi, lama sabachthani? (Which being translated is, "My God, My God, why did You forsake Me?") Psa. 22:1

The third hour Crucifixion time in Mark 15 corresponds to the sixth hour Roman time in John 19:14 because at the sixth hour John still has Jesus being interrogated by Pilate. At the sixth hour of John, Jesus was already on the cross (tree) according to the narrative of Mark 15:33. Mark timed the events in the third, sixth, and ninth hour. Those hours that are mentioned in the New Testament cannot be 3 AM, neither 6 AM, nor 9 AM, our Roman time. The consensus to take Jesus to Pilate happened in the morning which is at least after the third hour mentioned in the narrative, a time which cannot correspond to 3 AM.

Mark 15:1 KJV
(1) And straightway in the morning the chief priests held a consultation with the elders and scribes and the whole council, and bound Jesus, and carried him away, and delivered him to Pilate.

Luke 22:66 KJV
(66) And as soon as it was day, the elders of the people and the chief priests and the scribes came together, and led him into their council, saying,

Matthew 27:1-2 KJV
(1) When the morning was come, all the chief priests and elders of the people took counsel against Jesus to put him to death:
(2) And when they had bound him, they led him away, and delivered him to Pontius Pilate the governor."

We find John at the Crucifixion scene where Jesus gave him charge of His mother (John 19:25-27), right before He expired, at the ninth hour, Jewish time. He could not have mixed up the timing of that moment in the narrative.

John 19:25-27 KJV
(25) Now there stood by the cross of Jesus his mother, and his mother's sister, Mary the wife of Cleophas, 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 that disciple took her unto his own home."

The narrative implies that there is a three-hour difference in time reckoning between the Jews and the Romans. The Jews are three hours behind in telling their time, as mentioned earlier, because their day start in the evening while it is still daylight. Tabulating the time mentioned in Mark and John shows that the Jewish day starts at around 4 AM Roman time.
Therefore, the Sabbath day starts and ends at around 4 o'clock (or 5 o'clock for daylight savings time) Jerusalem time, wherever our geographic location permits allowing evening to fall at that time