In older court records, a petition to begin the probate process won't always indicate when the deceased passed away. However state statute dictates how soon a will must be presented to probate and various time restrictions. Usually a will is brought within thirty days of the day the person passed away, but there can easily be exceptions. Contemporary state statute should lay out the time frame.
But never use the date a will was proved in court as a death date. It can and should be used as a "dead by" date.