Kar Yong has reminded me about the genitive of time. I told him I have a faint memory of it either having discussed over it during bible study some years back or more likely during Greek class back in 2004, because had it been during the bible study, I would have remembered it quite well since the bible study I attended previously was serious business and at times we'd have it run pass 11.00pm!
I have just given myself a revision as to what genitive of time is, in Daniel B. Wallace's Greek Beyond the Basics (Zondervan, 1996, p.122-124). According to Wallace, "the genitive substantive indicates the kind of time, or time within which the word to which it stands related takes place." It is to "relate the genitive back to its basal significance," - of quality, attribute, description, or kind.
In the next 3 chapters, he basically repeats what he just said. I have no idea why he did that.
Let me try to understand this:
John 3:2 reads, 'this man came to Jesus by night and said to Him, "Rabbi, we know that You have come from God as a teacher; for no one can do these signs that You do unless God is with him.''' (NASB)
If it had not been a genitive, John would have meant that Nicodemus came in the night at such and such a time to see Jesus: it is more like "by the way, he came at night." It would not have made a difference if John hadn't mentioned that he came in the night, what matters was that Nicodemus came.
But because it is a genitive, it emphasize the kind of time that Nicodemus came, and the kind of time here happens to be at night, not morning or afternoon or evening, but in the dark of night. Not only did Nicodemus came, he specifically came in the night.