Limitation periods are decided as much by when the clock started as by how long it runs.
Accrual. Generally when the claim is complete — for contract, at breach; for tort, when injury occurs. Some claims accrue only on damage, which can be much later.
The discovery rule. In many states and for many claims, accrual is deferred until the plaintiff knew or reasonably should have known of the injury and its cause. Its availability varies by claim type and is often statutory.
Continuing violations and continuing wrongs. A series of acts may restart the period, or each act may generate its own claim with its own period. The distinction determines how much of a long-running dispute is recoverable.
Tolling. Minority, incapacity, the defendant’s absence from the state, fraudulent concealment, pendency of a class action, and agreements to toll. Tolling agreements should be in writing, specify the claims covered, the parties, the period, and the effect of termination.
Statutes of repose run from a fixed event regardless of discovery and are substantive rather than procedural, so they are usually not tolled.
Contractual shortening is enforceable in many states for many claims if reasonable, and is common in construction, insurance and technology contracts. Statutes prohibit it for some claims, and a clause shortening a statutory employment claim period is frequently unenforceable.