For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.
And he began to teach them, that the Son of man must suffer many things, and be rejected of the elders, and of the chief priests, and scribes, and be killed, and after three days rise again.
Jesus answered and said unto them, Destroy this temple, and in three days I will raise it up.
I would agree that a portion of a day would count as a day, meaning I don’t believe it would have to consist of all twelve (12) or twenty-four (24) hours to be consider a day.
And counting of the days is easy.
Friday end of day (1) + Saturday all day (1) + Sunday early morning (1) = 3 days.
Where the problem lies is in how one reads Matthew 12:40. Whereas the other scriptures claim “after three days” and “in three days”, this verse adds “and three nights”. It would be easy to dismiss the “and three nights” to hold true to tradition, but should tradition override scripture?
We must remember that this is Christ speaking. Had He meant to say “For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be [three days] in the [grave, tomb, etc.]” He clearly would have. This is God in the flesh, not a mere man that rambles on with no intention in his speech.
Jonah was the only sign given to that generation concerning the death, burial, and resurrection of Jesus Christ [Luke 11:29; Luke 11:30], and Jonah as in the whale “three days and three nights” not just “three days”.
Here lies the problem.
Friday all night (1) + Saturday all night (1) = 2 nights.
By this I suppose we have four choices:
1] Christ didn’t mean exactly what He said.
2] All Bible translations are in error concerning this verse.
3] Our tradition supersedes scripture.
4] Admit error.