The presentationTime read-only property of the LargestContentfulPaint interface returns the timestamp when the painted pixels were actually drawn on the screen.
The presentationTime is optional — some browsers may choose always to return 0 or not to expose the value at all. The value is also implementation-dependent — it may differ across browsers that choose to expose it.
A DOMHighResTimeStamp or null if the value is not exposed.
See Observing separate paint and presentation timings.