Cal. PEN § 4500

Every person while undergoing a life sentence, who is sentenced to state prison within this state, and who, with malice aforethought, commits an assault upon the person of another with a deadly weapon or instrument, or by any means of force likely to produce great bodily injury is punishable with death or life imprisonment without possibility of parole. The penalty shall be determined pursuant to the provisions of Sections 190.3 and 190.4; however, in cases in which the person subjected to such assault does not die within a year and a day after such assault as a proximate result thereof, the punishment shall be imprisonment in the state prison for life without the possibility of parole for nine years.

For the purpose of computing the days elapsed between the commission of the assault and the death of the person assaulted, the whole of the day on which the assault was committed shall be counted as the first day.

Nothing in this section shall be construed to prohibit the application of this section when the assault was committed outside the walls of any prison if the person committing the assault was undergoing a life sentence and was serving a sentence to a state prison at the time of the commission of the assault and was not on parole, on probation, or released on bail pending an appeal.