I am unable to change the due date of my ticket

Issue

When the due date of a ticket is changed, either manually or by a workflow rule, the due date changes back within a few minutes.

Cause

There is an SLA on the ticket that is set to Use SLA to automatically set the ticket due date and time. If this setting is applied, the due date of the ticket equal to the Resolved goal of the ticket. Even though it will appear as though you can change the due date/time and save it, after a few minutes, the system will automatically change it back to what was applied by the SLA originally.

Resolution

If you update the ticket status to a status that is mapped to the Waiting Customer SLA event, the upcoming SLA goals will be paused and the date can then be manipulated. When the status is updated to one that populates the Resolved goal, the due date will change to match.

Additional Resources