When Does the Clock Start: Incident or Discovery?
Every limitation period needs a start date, and the start date is contested far more often than the length. Two rules compete, and which one applies can be the whole case.
General information, not legal advice. Which rule applies to a particular claim is a question of local law.
The default: the clock starts when the harm happens
The ordinary rule is that a limitation period runs from the incident — the accident, the breach, the day the wrong was done. It is simple, it is predictable, and it produces a deadline anyone can calculate from a single date.
It also produces an obvious injustice in one category of case: harm that nobody could reasonably have known about at the time.
The discovery rule
Where it applies, the discovery rule starts the clock when the harm was discovered, or reasonably should have been. The classic examples are latent injuries that surface years later, defects hidden inside a structure, and fraud that stays concealed.
The statute of limitations calculator takes both dates for exactly this reason. Give it an incident on 10 January 2024, a discovery on 1 June 2025 and a two-year period, and it runs from the discovery date to produce a deadline of 1 June 2027 — nearly eighteen months later than the incident-based answer.
The calculator will not let discovery precede the incident
A small guard worth knowing about: a discovery date earlier than the incident date is rejected rather than silently used. You cannot discover harm before it occurs, and a tool that accepted the input would produce a deadline earlier than the incident-based one — the opposite of what the rule is for.
If you find yourself wanting to enter such a pair, the likely explanation is that the “incident” date you have is really the date of an earlier related event, and working out which date is the legally relevant one is exactly the sort of question to take to a professional.
“Reasonably should have discovered” is doing the work
Where the discovery rule exists, it almost never means actual subjective knowledge. It usually means the point at which a reasonable person in the claimant's position would have been on notice — which can be considerably earlier than the day they actually understood the problem.
This is why the discovery rule is a poor thing to rely on and a good thing to raise with a lawyer. It rarely gives as much extra time as it first appears to, and the analysis is fact-specific.
When the clock can pause
Separately from when it starts, a running period can sometimes be paused — tolled — by circumstances such as the claimant being a minor or incapacitated when the claim arose, the defendant concealing the harm or leaving the jurisdiction, or a written agreement between the parties to extend.
Tolling rules are technical and vary a great deal. Treat any of these as a reason to ask a professional rather than as a promise that your clock is paused.
Assume the earliest plausible start
The practical posture, when the start date is genuinely unclear, is to work from the earliest date that could arguably apply. If the deadline computed that way has not passed, you have time; if it has, you need advice about whether a later start or a tolling argument is available.
The reverse approach — assuming the most favourable start and relaxing — is how valid claims expire.