Which is flexible: Deadline or Definition of Done?

One piece of advice I frequently give my team is that on any project, one of two things is flexible:

• The deadline, when the project is shipped.
• The definition of done, which includes both quality and scope.⊕ Technically, there is a third dimension: the resources allocated to a project; however, engineers have a harder time changing that dimension.
  
  

It is important to always know which of these two applies for your project.

If the deadline is flexible, you can update stakeholders and let them know a project is slipping the delivery date when something goes wrong.

If the definition of done is flexible, you should be prepared to cut features or corners if necessary to complete a project on time.

On your project, which is flexible: deadline or definition of done?