The clock starts ticking on the statute of limitations when you discover that you suffered harm due to delayed treatment. You can call this the knowledge date. On this date, you discover that you suffered further illness or harm, the harm or progression were significant, and there is a clear link between late treatment and the progression/harm.
There are special cases that extend the time you have to file a lawsuit beyond two years. A good example of these circumstances is when the patient is under 18. In such a case, the two-year litigation window starts ticking when he or she reaches 18.