What is a timely action against an attorney for a wrongful act or omission?

An "action against an attorney for a wrongful act or omission, other than for actual fraud, arising in the performance of professional services" is timely only if filed "within one year after the plaintiff discovers, or through the use of reasonable diligence should have discovered, the facts constituting the wrongful act or omission or four years from the date of the wrongful act or omission, whichever occurs first." ( 340.6, subd. (a); see Adams v. Paul (1995) 11 Cal.4th 583, 589, fn. 2 ["discovery of the negligent act or omission initiates the [one-year] statutory period"].)