Body decomp.
Apple watch?
I believe that measuring the temperature of the brain is considered to be the most precise as the decline in brain temperature after death is very predictable. I don't know if this method is accurate to the minute, though. Someone has mentioned an Apple watch. That sounds very plausible. Because it monitors heart function, it would show when the heart stopped and I'm sure to the minute.
Made up!
Broken watch or phone I would bet.
Based on my vast experience watching murder mysteries on tv like Colombo