Understandably you are anxious to see dirt moved and walls erected. That move-in date can't come too soon. However, in the initial stages of the project, the delivery date is a moving target because of factors beyond a builder's control.
Weather is one obvious factor.
The process varies and can take a few minutes to many weeks depending on local practices and workload. This revision can take from several days to several weeks, and it must be completed prior to applying for a building permit. These groups generally meet only once a month. Your builder recognizes that timing is critical to planning your move. Although a guaranteed date is unrealistic in the early stages of construction, the builder can provide regular updates.