If they die more than a year and a day after the incident, not murder, if less, could be?
A man has been charged with murder following the death of a woman who died 21 years after being set on fire.
The year and a day is archaic? A guide not a rule? An invention by novelists?
https://www.legislation.gov.uk/ukpga/1996/19
Rule abolished in 1966.
I think it was connected with better methods of keeping victims alive.
The Great Train Robbery: the driver was beaten so badly he never worked again and died 18 months later. No murder charges could be brought. I think they never identified who had hit him.
Sorry, 1996.
In addition to what El Draque says above, medicine has advanced a lot so that now we can often tell what exactly killed someone. Previously it would have been unreasonable to claim that some action had definitely resulted in the death if the time between them was too great.