Expire Date- When we enter expiry date, it is save as prism:expirationDate metadata property on jcr node.
It means:-The date (potentially including time) by which the resource must be removed from availability to users or customers according to a rights agreement.
Prism is - Publishing Requirements for Industry Standard Metadata , it governs standard for Usage Rights Namespace.
You can find more information about it here :- http://www.prismstandard.org/specifications/2.1/PRISM_usage_rights_namespace_2.1.pdf
Expiry Date is like resource is expired permanently where as On/off time is the interval when your asset is activated or deactivated. You can on/off an asset multiple time but it expire only once.
Hope this clarify your doubt.