I do understand the indifference -- the deadline is more or less completely arbitrary, as there are very few critical scheduling issues dependent on strictly punctual delivery, and all that is really required is reasonable timeliness. If the professors allow it, why not use the additional latitude to your advantage?

Hard deadlines are generally driven by what are ultimately financial/market concerns. I sweat the deadlines when a trade show is approaching, and I sweat the budget when we see schedule slippage on an release, but most of the time I'm happy as long as everyone is productive, progress is being made, and the budget looks OK.