Project Duration vs Schedule Duration

The Properties tab shows the project start and end dates, and the schedule shows a start and end date, which is often mistaken for the project start and end dates. When tasks are created, the schedule duration will change to accommodate the task dates. The important thing to note here is that the schedule's dates show the entire schedule's duration but may not always correspond to the project's duration. 

The actual project duration is indicated by two dummy milestones named "Start Date" and "End Date", set to the project's start and end dates in the Gantt. They reflect the dates on the project's Properties tab at all times. You are free to keep them as a reference, rename, move, or even delete them as you see fit.

Changes to the schedule duration will affect the project duration only in certain situations. 

When you set the beginning or end date of a task in the project's schedule to dates outside of the existing project duration, the project start and end dates on the Properties tab automatically adjust. 

Further, updating a child project's schedule will not update the parent project's Properties tab dates. It'll only update the current project's properties. 

To explain this, imagine you are in the Gantt view of a parent project, and you extend the duration of one of the child project tasks to a date beyond the existing end date of the root initiative. You'll notice that the project's end date and all the header items above is replaced with the task's end date. At this point, the Gantt view gives the impression that all parent projects end on the same day as the task. Whereas, if you navigate to the Properties tab of any parent, you'll notice the dates haven't been updated. It'll only update the current project's properties. 

For any project's Properties tab to get updated, the task must be in its own schedule. The updated dates of the heading level items in the Gantt view is just a mechanism to show the total duration of scheduled activities at each level. In actuality, child projects can exist outside of the parent project span. 

Baselining

The interlinking of dates (or the lack of it) can also affect the baselining. It is a limitation, but if you want to baseline project start and end dates, the dates must be visible in the Properties tab. If you update the schedule in a child project causing the overall portfolio to extend beyond the originally planned end date, but the baseline is taken from a higher level in the hierarchy, the baseline will not capture the desired date change as the Properties tab at the higher level does not reflect the schedule extension at a lower level. You'll have to update the Properties tab manually before taking the baseline if you want the baselined portfolio end dates to reflect the schedule extension.

But having said that, what gets baselined when you update a task end date at any level in the hierarchy and use the Save & Update baseline button is the baselined end date for the task (This is reflected in the Baseline End Date column in Gantt) and if it stretches the direct parent project, the baselined end date for the summary project task in the schedule. If the project schedule dates get extended, the date on the Properties tab will be updated, causing the baselined project end date for the parent project to be updated.