As I prepare to write my first mystery, I've determined the weapon and the cause of death but I need my character to be caught standing over the body to make her a suspect. The best proof of her innocence will be the "Time of Death."
Here are three sources that can be used to determine the time of death:
1. Witnesses- the last persons to see the deceased alive.
2. Postmortem changes to the body
a. temperature of the body
b. the degree of rigidity (rigor mortis)
c. the degree of discoloration of the skin (livor mortis)
d. the degree of decomposition of the body
e. chemical changes in the eye
3. Associated events
a. scene "markers" (newspapers, TV schedules, letters)
Estimated time of death is the time when death actually aoccured.
Legal time of death is what's recorded on the death certificate and is the time at which the body is discovered.
Estimated time of death is one of the most important factors in a murder case. This time can be used to convict a murderer, beak an alibi or eliminate a suspect(How I hope to prove my heroine innocent). However this is difficult to determine because of so many things that can affect the rate at which the postmortem changes develop. Due to these changes it is easier to determine the time of death the faster a body can be after death has occurred. An exact hour and minute can't be determined unless there is a witness.
Source: The Howdunit Series- Cause of Death: A writer's guide to death, murder, and forensic medicine by Keith D. Wilson, M.D.