In New York there is no recognizable discovery rule for the statute of limitations.  For the most part, the statute starts to run on the day of the mistake, although it may be tolled for continuous representation, or fraud [which does not consist of merely hiding the malpractice], but in Delaware there is a specific rule.

Here is a case which discusses the rule and its application. Boerger v. Heiman, Superior Court of the State of Delaware.