There is no possible way to know the exact time of death forensically. Even in the best of circumstances and a liver temperature taken very soon after death it is simply not possible to be more accurate than a time frame window of a couple of hours or so. Considering that Stephany was not found until days later the most accurate time frame of death would have to have been figured out by the time she was last heard alive in the room (argument had allegedly occurred) and the time Joran left the hotel to begin his run from the police.
http://www.writersdigest.com/article/forensics-excerpt
"But most deaths are not witnessed. Natural death may come during sleep, and accidental and suicidal deaths often occur when the victim is alone. In homicides, the perpetrator is typically the only witness and he rarely checks his watch, and even if he did, hes not likely to talk about it. This means that when the medical examiner must determine the time of death he can only estimate the approximate time.
These times of death may differ by days, weeks, even months, if the body is not found until well after physiologic death has occurred."
