Offline days, when greater than 0, allow Webviewer user to click the toolbar icon "Save to Browser" and save that content to the browsers cache and be offline -- no Internet access -- for those number of days. The user would also have to bookmark the Webviewer URL in order to access that content offline.  Once the number of days lapses, if the user is online, they will automatically be authenticated once again and be granted another n days offline. If the user is offline, the content will be locked and the user logged out. The user will need to be online and login once again to continue to access the content. This re-authentication forces the Webviewer content to communicate back with Vitrium’s server to re-validate the User’s authenticity while checking if the content and/or DRM policy is still valid.

Expiry date is when a content is set to expire, depending on the level the DRM Policy is set, it renders all access for the User(s) or Group(s) to be denied when the date comes. This is determined by setting a specific date of expiry. 

Thus, the most predominant difference is that Offline days used for those users can’t connect to the internet and this feature enables them to have access to the protected content without internet connection. On the other hand, Expiry dates limits user(s) or group(s) from accessing the content after the date you’ve set. Subscription-based businesses can benefit from Expiry date feature and also expiry after first unlock.

NOTE: If Expiry date is set, offline days will be shortened if it allows the user to go beyond the expiry date.