If the previous document and the current document have the same origin, the IHTMLPerformanceTiming::unloadEventStart property must return the time immediately before the user agent starts the unload event of the previous document. If there is no previous document or the previous document has a different origin than the current document, this attribute must return zero.
HRESULT value = object.get_unloadEventStart(ULONGLONG* p);
An ULONGLONG value that represents the time immediately before the user agent starts the unload event of the previous document.
- Navigation Timing, Section 4.2
The value reported by the IHTMLPerformanceTiming::unloadEventStart property represents the number of milliseconds between the recorded time and midnight January 1, 1970 (UTC).
Windows Internet Explorer 9. This property is supported only for documents displayed in IE9 Standards mode. For more information, see Defining Document Compatibility.