The early start, early finish, late start, and late finish dates of a project schedule are the primary dates that are calculated in any project schedule. The early start dates of the project schedule are the earliest that any activity in the schedule can be scheduled to start given the logic and constraints of the schedule. The early finish of an activity in the schedule is the earliest that the activity can be scheduled to be completed given the logic and constraints of the schedule. The late start of an activity is the latest that a project activity can be scheduled to be started without having to reschedule the calculated early finish of the project. The late finish of an activity is the latest that a project activity can be finished without having to reschedule the late finish of the project. The late finish of the project is the late finish of the last activity to be completed in the project.
The schedule that is made up of the early start and early finish of each activity in the schedule is called the early schedule. The schedule that is made up of the late start and late finish of each activity in the schedule is called the late schedule.
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The early start of the project is declared first. Unless we have at least one date that tells where the project begins or ends, it will be possible to know the schedule only relative to other activities. Normally we want to know the project schedule relevant to real calendar days over a specific period of time. All activities that have no predecessors must have a declared start date. In most of the project management software available today, this information is entered as the project start date and is used for any activity not having a predecessor. Most software also allows the calculation of a schedule using a declared project finish date as well. For our discussion let us say that we will be declaring the date that we will start the project.
Once we have declared the beginning of the project, we can say that the early start of each of the activities without at least one predecessor will have an early start equal to the declared project start date.
By convention and agreement between schedulers, it is assumed on project schedules that work always begins in the beginning of the work period that the activity starts in and always ends at the end of the work period that the activity ends in. This means that if an activity is scheduled in days on a one-shift basis, which is the true for most project work, the activity is scheduled to start in the morning of the day it is scheduled to start and finish in the afternoon of the day it is finished. This means that an activity that has a one-day duration would start in the morning on Monday, January 15, and finish in the afternoon on Monday, January 15. An activity having a two-day duration starts on January 15 and finishes on January 16. This creates some confusion since many people would expect that the difference between the early start and the early finish of an activity should be equal to its duration, and this is not the case. The early finish of an activity is the early start of the activity plus the duration minus one.
In the case of the early schedule, the early start of the successors of any activity is the beginning of the next time period after the completion of the preceding activity. If an activity finishes on January 25 and is the predecessor to another activity, the successor activity will be scheduled to start on January 26.
Notice that in the early schedule if a successor activity has more than one predecessor, the early start of the successor activity is calculated as the time period following the latest of all of the predecessor early finishes.
The early finish date of these activities is calculated by taking the early finish date of the activity and adding the duration to it. When this is done, some care must be taken to ensure that the duration is added to the early start date and that the weekends, holidays, and other nonworking days are also added to calculate the early finish date.
The late finish date must have a way of getting started as well. In the case of project schedules, the late finish of the project is set equal to the early finish of the latest early finish date of any activity in the project. Every activity in the project schedule that has no successor is given this date as its late finish. If we had five activities that had no other activities logically following them (no successors), each of them would have a late finish equal to the early finish of the last activity to be completed on the early schedule.
To get the late start of each activity, we must subtract the duration from the late finish of the activity. Again, as with the calculation of the early schedule care must be taken not to include weekends, holidays, or other nonworking days. As with the calculation of the early schedule, we must be careful to adjust the scheduled late start date because of the convention that all activities will start on the morning of the day on which they start and finish on the afternoon of the date on which they finish. This means that an activity that has a late finish date of January 20 and a duration of three days has a late start date of January 18.
In the case of the late schedule, the late finish of the predecessor of an activity is the end of the last time period before the succeeding activity is scheduled to start. If an activity is scheduled to have its late start on February 7, all of its predecessors must be completed not later than February 6 in order to allow it to start as scheduled. Notice that in the late schedule, if a successor activity has more than one predecessor, the late finish of each of the predecessor activities is calculated as the time period before the late start of the successor’s late start. This just says that no activity can finish later than the date necessary to support the late start of its successor.