Each box in a network diagram must contain the duration of each task, as well as a position for the early start, early finish, late start, and late finish for the node. What is the definition of early start, or ES, in each of the nodes of a network diagram? The early start, or ES, is the earliest duration of the activity. When determining the critical path, ES is the earliest possible point in time when the activity can start, based on the network logic of the prior activities. In the critical path determination, ES is the early start date of the project