There are some contexts in social networks in which obsolescence of knowledge of a person regarding some topic of interest should be considered when establishing trust between parties. However, and as of this writing, obsolescence of knowledge has not been considered as a dimension in trust models. In this work we propose to consider obsolescence of knowledge in trust models. Two studies were run in order to explore the relation between trust and obsolescence of knowledge. Results from these studies show that trust is decreased as obsolescence is incremented and that the relation is not linear.