Activity Dates

When a project is scheduled two main sets of dates are calculated for activities:

Early dates are the earliest time the activity can happen according to the schedule logic.
Late dates are the latest time the activity can happen without jeopardizing the end date of the project.

The early start and early finish dates for an activity are calculated according to durations, calendars, relationships, and constraints for the activity and all predecessor activities.

The starting point for calculating early dates is the data date for the project. The early start date for all activities without predecessors is calculated as the data date for the project. The early start date for an activity with predecessors is usually calculated as the latest early finish date of all predecessor activities.

The early finish date for activities which have not started is usually calculated as the early start date plus the planned duration for the activity according to the appropriate calendar. The early finish date for activities which have started is calculated as the early start date plus the remaining duration for the activity according to the appropriate calendar.

However, because dates are calculated according to the logic of the schedule, the relationship an activity has with its predecessor can affect how the early start and early finish dates are calculated:

The early start date of a not started activity is calculated:
for start-to-start relationships, the start date of its predecessor.

for finish-to-finish relationships, the finish date of its predecessor minus the activity duration.

for start-to-finish relationships, the start date of its predecessor minus the activity duration.
The early finish date of a not started activity is calculated:
for start-to-start relationships, the start date of its predecessor plus the activity duration.

for finish-to-finish relationships, the finish date of its predecessor.

for start-to-finish relationships, the start date of its predecessor.

Late dates are similarly calculated from the end of the project and working backwards. The end of the project is determined as the latest early finish date of all activities, unless the Must Finish By constraint has been applied to the project in which case that date is used as the end date of the project.

The calendar used for calculating the date of an activity depends on the activity type. Task dependent activities are scheduled according to the activity calendar. Resource dependent activities are scheduled according to the calendars for the resources. If multiple resources are assigned to a resource dependent activity, the latest finish date of all driving resources is used for the early finish date of the activity.

Tips

The remaining early start and remaining early finish are the same as the early start and early finish respectively unless you are using the Preserve scheduled early and late dates option with resource leveling.
Resource leveling can change the early and late dates for an activity. If a resource is overallocated activities are delayed until the resource has time to complete the work.

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