I like how you slipped "modern thinking" into that...
According to the Creator of the Heavens and Earth I measure a day;
Berĕshith/Genesis 1:5, " And Yah called the light ‘day’ and the darkness He called ‘night.’ And there came to be evening and there came to be morning, the first day."
Exodus 12:18, "In the first month, on the fourteenth day of the month, in the evening, you shall eat unleavened bread until the twenty-first day of the month in the evening."
Leviticus 23:32, "It is a Sabbath of rest to you, and you shall afflict your beings. On the ninth day of the month at evening, from evening to evening, you observe your Sabbath.”
"evening" is word # H6153. ereb - Strong's Concordance: ereb: evening, Original Word: עָ֫רֶב, Part of Speech: Noun Masculine, Transliteration: ereb, Phonetic Spelling: (eh'-reb), Short Definition: evening
Hebrew Word Study (Transliteration-Pronunciation Etymology & Grammar) -
1) evening, night, sunset, 1a) evening, sunset, 1b) night
Once we know what a day is according to Father YHWH we can
1. Understand the difference between His day and any other measurement of a "day"
2. I think it is same to assume that Yahshua/Jesus followed His Father's day and not the "day" of the pharisees
3. We can look at His words directly;
Mat 12:40, “For as Yahnah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.”
Why wouls He say the only sign of the true Messiah is, "three days and three nights in the heart of the earth"
He made a very clear distinction "three days and three nights" If He had just said 3 days, possibly, but He made it very clear.