Lead Time and Lag Time
Lead time and lag time are changes in precedence relationships that direct a delay or acceleration in the successor task. Both are formal ways to adjust the schedule.
Lag time is a delay imposed on the relationship between two activities. For example, in a finish-to-start dependency with a 10-day lag, the successor activity cannot start until 10 days after the predecessor has finished. Lag time is generally expressed as a positive number because lag time adds to the overall duration of a series of activities. Lag time must be counted when computing your schedule duration.
Lead time is an acceleration of the successor activity. For example, in a finish-to-start dependency with a 10-day lead, the successor activity can start 10 days before the predecessor has finished. Lead time is usually expressed as a negative number because it subtracts from the overall duration of a series of activities.
An example of the use of lag time is adding one day to a six-day model airplane project schedule. After the first three-day activity, Glue Model (activity 1.1), is complete, one day of lag time is added to enable the glue to dry. Then the remaining three-day activity, Paint Model (activity 1.2), can be accomplished. The result is an overall schedule duration of seven days.
Lag time is not counted as part of an activity's duration. Lag represents a fixed delay between the start or finish of one activity and the start or finish of another.