Changing the start date does not change the start date. Normally start
dates are calculated based on the project start date, the links between
tasks, and the durations of the predecessors to the task in question. If
you enter something manually in the task's Start Date field, you are
actually setting a 'Start No Earlier Than...' constraint, not the start date
per se. Project's job is to calculate the schedule for you - you don't tell
it the dates you want tasks to start on, it tells YOU the dates tasks CAN
start on. You use that to direct the resources as to what they ought to be
doing and when they ought to do it. Let it do its job for you.