Once you've figured out what statute of limitations applies to your case, your next step is to determine when the clock starts ticking. In most situations the time starts to run on the "date of harm" -- that is, on the date when you were injured, your property was harmed, or a contract or agreement was violated.
However, a huge exception to this general rule exists. The exception protects plaintiffs in situations where they may not be aware for months or even years that they have been harmed. In such situations, statutes of limitations may start the clock ticking either on the "date of discovery" of the harm or on the date on which the plaintiff "should have discovered" the harm. In short, for some types of legal actions the statute of limitations clock can start ticking at three different times!
Example 1: On January 1, a doctor performs a gallbladder operation on Phoebe but mistakenly removes Phoebe's spleen. The doctor tells Phoebe of the screwup as soon as she wakes up. Phoebe's time period for suing the doctor begins to run on January 1, since the harm occurred on that date and Phoebe actually knew about it. If a two-year statute of limitations for medical malpractice applies to Phoebe's case, she'd have two years from January 1 to file a lawsuit against the doctor.
Example 2: Same case, except the doctor tells Phoebe nothing of the surgical screwup. Phoebe is in constant pain following the January 1 surgery. A month later, on February 1, Phoebe talks to another doctor who tells her that she should not be in pain and that she should immediately come in to have it checked out. Phoebe delays going to the doctor until July 1 of the same year, at which time she finds out that her spleen had been removed mistakenly on January 1. In this situation, Phoebe's time period for suing the doctor probably begins to run on February 1, because the pain coupled with the second doctor's advice determines when Phoebe reasonably should have discovered the harm.
Example 3: Same case, except that Phoebe suffers no unusual after-effects following the January 1 surgery. Phoebe is unaware that anything went wrong with the surgery until July 1 of the same year, when an X-ray during a routine medical checkup reveals that her spleen was removed. In this situation, since Phoebe did not discover and could not reasonably have discovered the harm until July 1, most states would measure Phoebe's time to sue from July 1.