Doctrine and Covenants 20:1; “The rise of the Church of Christ in these last days, being one thousand eight hundred and thirty years since the coming of our Lord and Savior Jesus Christ in the flesh, it being regularly organized and established agreeable to the laws of our country, by the will and commandments of God, in the fourth month, and on the sixth day of the month which is called April—” [emp. mine]
Matthew 1:18; “Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.”
Matthew 2:1-3; “Now when Jesus was born in Bethlehem of Judaea in the days of Herod the king, behold, there came wise men from the east to Jerusalem, 2 Saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him. 3 When Herod the king had heard these things, he was troubled, and all Jerusalem with him.”
Joseph Smith’s revelation in D&C 20 proved false, as it was a result of ignorance regarding historical facts.
Numerous historians have documented Jesus was born shortly before the death of Herod the Great, and the Gregorian Calendar was off by four years. You can retrieve more info in the link above.
Moreover, the date of Herod’s death can be verified through the writings of Josephus who recorded an eclipse of the moon on March 12-13, 4 BC. Josephus also noted Herod died just before the Passover on April 11, 4 BC.
Based on these evidential facts, we can see that Jesus’ birth probably took place sometime in March 4 BC, which makes Smith’s revelation false.