Forty Seconds for the Cause, Six Minutes Forty Seconds for the Details
Cause of Death · Timing Window
After a name is written, the writer has forty seconds to add a cause of death. If no cause is supplied, the victim dies of a heart attack. Once a cause is written, another six minutes and forty seconds are available to add the circumstances.
Why it matters
The Death Note is not merely a list of names. Those windows are what let Light turn deaths into messages, tests and alibis instead of leaving only identical heart attacks.
The Death Note Cannot Make the Impossible Happen
Control Has Physical and Informational Limits
The writer can control a victim's actions before death only when those actions are possible for that person to perform. The notebook cannot make someone do something physically impossible or produce information they could not reasonably know. When the written circumstances cannot occur, the impossible instructions fail and the victim dies by heart attack instead.
Why it matters
Light's plans work because he writes around the victim's actual situation. The notebook gives him control, but it does not give the victim knowledge or abilities they never had.
The 23-Day Control Limit
Long-Term Control Has a Ceiling
When the Death Note is used to control a person before death, the planned death normally has to occur within twenty-three days of the name being written. Disease is the major exception when the specified illness naturally takes longer to run its course.
Why it matters
The notebook allows elaborate planning, but it is not unlimited long-range programming. Kira still has to build operations inside a finite window.