The IHTMLPerformanceTiming::navigationStart property must return the time immediately after the user agent finishes prompting to unload the previous document. If there is no previous document, this property must return the same value as IHTMLPerformanceTiming::fetchStart.
HRESULT value = object.get_navigationStart(ULONGLONG* p);
An ULONGLONG value that represents the time immediately after the user agent finishes prompting to unload the previous document.
- Navigation Timing, Section 4.2
The value reported by the IHTMLPerformanceTiming::navigationStart property represents the number of milliseconds between the recorded time and midnight January 1, 1970 (UTC).