Can you kill a human with a Death Note in less than forty seconds? If I were write in the Death Note:


  Bob Bold Heart attack NOW.


Would that work?
According to the first rule in How To Use III


  If the time of death is written within 40 seconds after writing the cause of death as a heart attack, the time of death can be manipulated, and the time can go into effect within 40 seconds after writing the name. 


So the time of death (e.g. "NOW") won't go into effect for 40 seconds.