These dates are projected dates. We will not know the true dates until certain things take place. The new moon must be sighted to officially start a month. The barley has to reach a state of maturity that falls between the agricultural terms of soft dough and hard dough. This is the maturity that is required to have a "Wave Sheaf" offering 2 weeks later during the Week of Unleavened Bread. Traditionally this state of plant maturity was called "AVIV". If one searches back even further you will discover the Passover Festival Week is called the "Time of Life" - the time of renewing covenant with YEHOVAH.