The option of Expiry Date occurs when one is creating a new gallery.

The expiry date sets a limit to the time that a client can view the photos. Once the expiration has reached, the client will no longer be able to view the photos.

In a way, the expiry date limits your client interaction with you on their photos and thus helps you close a project on time.

Further, this setting can be changed even after a gallery is made. Expiration date can be changed and even after a project has automatically closed, it can be reopened and the expiry date can be set on a further date to best suit your purpose.