The question of how Jesus Christ spent three days and three nights in the tomb is one that many believers, both new and seasoned, have grappled with. The confusion arises from trying to reconcile the timeline of Jesus' death, burial, and resurre. The "three days and three nights" mentioned in Matthew 12:40 have confused some people because of the supposed differences between how long Christ said He would be in the tomb as it relates to the amount of time between His burial and resurrection that are recorded in the Bible.
If the three days and nights were counted the way we count them, then Jesus would have to rise on the fourth day. But, by comparing these passages, we can see that in the minds of people in Bible times, "the third day" is equivalent to "after three days." In fact, the way they counted was this: part of a day would be counted as one day. Jesus Christ gave only one sign that He was the Messiah-He would be three days and three nights in the tomb.
If He wasn't, then, by His own words, He should be rejected as our Savior. But He was three days and three nights in the tomb! The sign was fulfilled exactly as He said. He is our Savior! How do you count three days and three nights? This is a very common question that is asked throughout Christianity.
Today when we talk of a day, it means 24 hours. Hence three days is 72 hours. As you point out, Jesus was apparently not in the tomb for 72 hours.
Christ died around three in the afternoon on Friday and was entombed shortly thereafter. The Resurrection occurred by dawn on Sunday. Thus he was in the tomb less than nine hours Friday (by the modern reckoning), twenty-four hours Saturday, and less than six hours Sunday-at any rate, far less than the seventy.
But once we understand how Jesus and the majority of Jewish people around Him understood days, the seeming contradiction vanishes. Jesus' death at 3 p.m. on Good Friday counted as one day, His entombment all day Saturday counted as the second day, and His resurrection on Sunday morning counted as the third day.
"The other statements, "in three days"... do not include the total time that he was dead but only the time that he was buried in the tomb" (The Day Jesus the Christ Died, Chapter 6). Was Jesus in the tomb for three days and three nights? (Matthew 12:40 vs.
Mark 15:42-47) Was Jesus in the tomb 3 days/nights? Jesus exited the tomb not long after the sun started rising. When we look at Jesus' act of salvation and its timing through the cultural context it was originally written in, we realize that He truly was in the tomb three days and three nights. Jesus' prophesy from Mark 12:40, like all of His other words, came true.