Schedule calculation

The schedule for a project and WBS elements refers to the start date, finish date, and duration information. In a WBS structure, the schedule for parent elements is determined by the schedule of the child elements.

Schedule fields

To specify a schedule, you need to enter any two of the schedule fields; the third field is calculated and displayed:
  • If you enter the start date and duration, the finish date is calculated and displayed.
  • If you enter the finish date and duration, the start date is calculated and displayed.
  • If you enter the start date and finish date, the duration is calculated and displayed.
When you modify a schedule fields, the dependent field is recalculated as described below:
  • If you modify the start date to an earlier start date, the duration is increased. If you modify the start date to a later start date, the duration is decreased. The finish date is not affected by either of these changes.
  • If you modify the finish date to an earlier finish date, the duration is decreased. If you modify the finish date to a later finish date, the duration is increased. The start date is not affected by either of these changes.
  • If you increase the duration, the finish date is postponed to a later date to accommodate the new duration. If you decrease the duration, the finish date is brought forward to an earlier date to match the new duration.

Default schedule

When you create a new element (project or WBS), a default schedule is created for it.
  • If the new element is a root element, for example, a new project or program that currently has no child elements, the default schedule is based on the system dates. The start date is the current date, and the duration is one day. The finish date is the same as the start date.
  • If the new element is a child element of an existing element, the default schedule is based on the parent element.
    • If the start date of the parent element is the current date or later than the current date, the default start date of the child element is inherited from the parent. The start date of the child element is the start date of the parent element, and the duration is one day.
    • If the start date of the parent element is earlier than the current date, the default start date for the child element is the current date, and the duration is one day. The finish date is the same as the start date.

Modification of the default schedule

To create a schedule, you need to modify the default schedule and enter your own start and finish dates, and duration for the elements.

When you add a child element to an existing element, the parent element schedule fields are disabled, and you cannot modify the parent schedule fields directly. The schedule of the parent is aggregated from the schedule of the child elements.

The rules for the calculation of the schedule fields for a parent element are:
  • The start date of the parent is the start date of the earliest scheduled child element.
  • The finish date of the parent is the finish date of the latest scheduled child element.
  • The duration of the parent is the available working days (as specified in the project calendar) between the start and finish dates that are aggregated from the child elements.

To modify the schedule of a parent element, you must modify the schedule of its child elements. Any change that you make in the schedule of a child element is aggregated to the parent element, and the parent element schedule is modified to reflect those changes.

When you modify the schedule of a task that has dependencies, the change is accepted only if it does not affect the schedule constraints of the dependent tasks. For example, assume Task A is dependent on Task B, Task B is dependent on Task C, and Task C is dependent on Task D. All the dependencies are Finish-to-Start, and Task D has a Finish no later than constraint. You modify the start date of Task A so that task D cannot complete by the schedule constraint date. You are prompted that the date change of Task A breaks the constraint on Task D, and you have the following choices:
  • Continue with the date change and automatically remove the constraint - If you select this option, the start date of Task A is changed and the constraint on Task D is removed.
  • Keep the constraint and do not accept the date change - If you select this option, no changes are made. The start date of Task A is not changed, nor is the constraint on Task D removed.

Example 1: Calculation of a project schedule

Based on the aggregation rules, the schedule for a project is calculated as given below:
  • The start date of the project is the start date of the earliest scheduled project element.
  • The finish date of the project is the end date of the latest scheduled project element.
  • The duration is calculated based on the available working days (as specified in the project calendar) between the project start and finish dates.

Example 2: Calculation of a summary task schedule and effort

Based on the aggregation rules, the schedule for a summary task is calculated as given below:
  • The start date of the summary task is the start date of its earliest scheduled child element.
  • The finish date of the summary task is the end date of its latest scheduled child element.
  • The duration is calculated based on the available working days (as specified in the project calendar) between the summary task start and finish dates.
Related concepts
Constraints
Dependencies
Related tasks
Assigning dates and duration to WBS elements
Applying constraints
Creating dependencies

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