The prophecy that Israel will be "born in a day" is for when Yeshua returns to earth.  Israel was not born in a day on May 14, 1948.  They were as alive and thriving on May 13, 1948 as they wre on May 14.  All that changed was that they said so.  What this prophecy tells us is that Israel must go into exile yet one more time before the return of the Messiah.