If the previous document and the current document have the same origin, the 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.
An Integer 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 unloadEventStart property represents the number of milliseconds between the recorded time and midnight January 1, 1970 (UTC).
The following example shows how to calculate the time required to unload the previously loaded document from the window object.
var oTiming = window.performance.timing; var iTimeMS = oTiming.unloadEventEnd - oTiming.unloadEventStart;