Early and Late Dates
Once the logic that creates a network has been established, the scheduling software can calculate a set of start and finish times for each activity, given the relationship logic and the estimated duration of each activity. Ideally, only one date should be entered into the scheduling software—the program start date. All other dates are calculated by the network logic. Network logic calculates activity dates that define both when an activity may start and finish and when an activity must start and finish to meet a specified program completion date. These are known as early and late dates, respectively.
Each activity has an early start date and an early finish date. Unless otherwise stated in this guide, “start” and “finish” refer to an activity’s early start and early finish dates.
Early start defines the earliest time when an activity may start.
Early finish defines the earliest time when an activity may finish.
The early start and early finish dates for each activity are calculated by the “forward pass” method. The forward pass determines the early start and early finish times for each activity by adding durations successively through the network, starting at day one. The forward pass will derive the total time required for the entire program by calculating the longest continuous path through the network—that is, the sequence of activities that determines the length of the program, typically known as the critical path. Managing the critical path is the foundation of Best Practice 6.
Each activity also has a late start date and a late finish date.
Late start defines the latest time when an activity must start in order for the program to be completed on time.
Late finish defines the latest time when an activity must finish in order for the program to be completed on time.
The late start and late finish for each activity are calculated by the “backward pass” method—the opposite of the forward pass. Whereas the forward pass determines early finishes by adding durations to early starts, the backward pass determines late starts by subtracting durations from late finishes.
The difference between an activity’s early and late dates is known as total float or slack. Total float is an essential output of critical path method scheduling, and its proper management is the foundation of Best Practice 7.
In figure 11, setting steel columns and beams may begin as early as November 12 (early start) and finish as early as November 21 (early finish). However, for the house construction project to be completed on time, setting steel columns and beams must begin by November 13. The activity therefore has 1 day of total float: the difference between its early start date of November 12 and its late start date of November 13. The blue box in figure 11 represents the early start and early finish dates, and the red box represents the late start and late finish dates.
Figure 11: Early and Late Dates
Appendix IV gives a detailed example of how to calculate early and late dates using the forward and backward passes.