What is the waiting period (iddah) for a woman who was divorced but did not learn of her divorce until three months later?

Scholars have differed regarding the starting point of the waiting period (iddah) for a divorced woman who did not know of her divorce until some time later. Is the iddah to be calculated from the time she learned of the divorce, or from the time the husband acknowledged it? The more preferable approach is for her to observe the iddah from the time she learned of the divorce, as a precautionary measure, to avoid disagreement.