Here are two witnesses that he is unclean until evening and when the sun is down, he shall be clean.  This aspect of evening appears to begin at sunset.

Deu 16:6 But at the place which the LORD thy God shall choose to place his name in, there thou shalt sacrifice the passover at even, at the going down of the sun, at the season that thou camest forth out of Egypt.
 

Mat 27:57 When the even was come, there came a rich man of Arimathaea, named Joseph, who also himself was Jesus' disciple:

Here, evening came well after Y'shua had died.  Perhaps process of the death on the tree was not complete until sundown.

Jos 8:29 And the king of Ai he hanged on a tree until eventide: and as soon as the sun was down, Joshua commanded that they should take his carcase down from the tree, and cast it at the entering of the gate of the city, and raise thereon a great heap of stones, [that remaineth] unto this day.

Psa 104:23 Man goeth forth unto his work and to his labour until the evening.

Dan 6:14 Then the king, when he heard [these] words, was sore displeased with himself, and set [his] heart on Daniel to deliver him: and he laboured till the going down of the sun to deliver him.

Do people labor until noon or three o'clock?  No they often labor until the sun goes down, which appears to be when evening begins.

In the Genesis account Day = light, Night = dark, and evening and morning appear to be the transitions between the two.  I agree that evening is never dark, but point out many witnesses that in a primary sense evening begins at sundown.

Gen 24:11 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].

The Genesis verse here modifies evening, with "time of the evening" which may signify a broader time when evening was approaching.

Jdg 19:9 And when the man rose up to depart, he, and his concubine, and his servant, his father in law, the damsel's father, said unto him, Behold, now the day draweth toward evening, I pray you tarry all night: behold, the day groweth to an end, lodge here, that thine heart may be merry; and to morrow get you early on your way, that thou mayest go home.

1Sa 30:17 And David smote them from the twilight even unto the evening of the next day: and there escaped not a man of them, save four hundred young men, which rode upon camels, and fled.

Here twilight may be the period of the evening breeze, but most likely after sunset.  Here it is not clear from the context when evening was.

Zec 14:7 But it shall be one day which shall be known to the LORD, not day, nor night: but it shall come to pass, [that] at evening time it shall be light.

Here it is implied that at evening time it is normally beginning to get dark