When you first create a task, its early start and early finish dates are the same as the scheduled start and finish dates. As you link the task to predecessor and successor tasks and apply any date constraints, the early start and early finish dates are calculated as the earliest possible dates this task could start and finish if all predecessors and successors also start and finish on their respective early start and early finish dates. If there is a leveling delay on the task, this delay is also figured into the early start and finish dates. These calculations are based on a fixed duration.