The time period calculation starts when the personal injury occurs. But it may not be so all the time. In technical terms, the important reference point is “accrual of claims”. This is the time when the injury or its cause is known. This “discovery rule”offers the facility to file a suit within a certain time after the injury is discovered or is supposed to be discovered.
Further, there are certain provisions for a minor (less than 18 years) and mentally-challenged plaintiffs at the time of injury. These provisions come in force when the “statute of limitations” is “tolled” (paused) for some time. It can also be reduced through contract.