What should you do if you are certain about having sworn an oath, but you doubt the details of this oath, such as the period for which you committed yourself?

If the one who swore an oath is certain that he restricted his oath by a specific time, but doubts the duration of that restriction, then he should assume the minimum certain duration. This is because a term is understood to mean the least of what it can refer to. Therefore, doubt about breaking the oath does not necessitate expiation, because the default is the exoneration of one's liability, and certainty is not removed by doubt.