The early start date used in the critical path method is the earliest possible date when the uncompleted portions of a schedule activity can start based on the schedule network logic, the data date, and any schedule constraint.

To calculate early start dates, we begin with the project start date and assign that date as the start date of the first activities (those that have no predecessor activities); and then move forward to calculate the early start dates of subsequent activities (having finish-start relationships) by adding the predecessor activity’s duration to its start date, then adding the lag time or subtract the lead time.