The IntersectionObserverEntry interface's read-only time property is a DOMHighResTimeStamp that indicates the time at which the intersection change occurred relative to the time at which the document was created.

A DOMHighResTimeStamp which indicates the time at which the target element experienced the intersection change described by the IntersectionObserverEntry. The time is specified in milliseconds since the creation of the containing document.

See Timing element visibility with the Intersection Observer API for a complete example which uses the time property to track how long elements are visible to the user.