To many scholars this is describing that when Jesus ascended he took with Him those who rose on the day of First Fruits with Him. That being so, then we have Jesus casting a shadow to be fulfilled by Messiah. Well, lets consider Paul's prophecy to the Thessalonians. These folks got a sign - most likely visible to all who cared to see or listen to those who knew that they had seen dead walking. They had to be among the first to accept Jesus on that Pentecost morning when Peter and the apostles began to preach Jesus as the Messiah to the Jews.
Lets go one step further.
I believe that the 40 days of Jonah's warning was the shadow cast to be fulfilled on First Fruits by Christ, who recasts the shadow into the future to be seen again 40 days before the Rapture, when those that rise will join together with those who are alive and remain to be caught away. And, that all occurred in the Spring of the year the first time, and I have no doubt that it will occur in like manner and time when the time comes. So, keep looking at the Jews and Jerusalem for the timing. And to remind you that it also says that when that happens there will be much new knowledge. Please get that straight.