The project manager is unable to change the project start or finish dates. If the dates are updated as soon as the project manager clicks on Save, the dates revert back to the original dates. However, a user with full administrator rights can change the dates without an issue.
Project Start Date is 7/1/2018; it needs to be changed to a later date of 8/1/2018.
Project Finish Date is 10/25/2019; it needs to be changed to an earlier date of 9/25/2019.
Grant the Project Manager Resource - Hard Book rights so that they will be able to update the project finish dates. If the user has Hard Book rights, changing the project start/finish dates will trigger a change to the hard allocation start/finish dates automatically. (If the user does not have Hard Book rights, this same change to the hard allocation finish dates will be attempted when the project dates are updated, but this change cannot be completed due to lack of access rights, so the project dates as well as the allocation dates will remain unchanged.)
Have the Resource Manager (or another user that already has Hard Book rights) change the allocation start and/or finish dates of the team members to be within the target project dates. Then the Project Manager will be able to make and save the changes to the desired project dates.
Note: If the project manger does not have hard book rights, the project start date can be changed to an earlier date and the Hard Allocation start dates will remain at the original later date. Likewise, the project finish date can be changed to a later date and the Hard Allocation Finish dates will remain at the original earlier date. In both of these scenarios, the Hard Allocation dates are not changed, but they are still within the new project start and finish date range, so they do not prevent the project manager from making the updates to the project dates.