Whether the tenants' restitution claim for sewage-tax charges was time-barred under Art. 67 CO.
The one-year limitation period began only when the tenants actually learned of the true legal situation on 11 December 2008; the action filed in April/July 2009 was timely.
For unjust enrichment claims, the period runs when the creditor has sufficient certainty about the claim, not when they merely could have discovered it. The landlord failed to prove earlier knowledge by the tenants.