Querying large databases containing imperfect data requires efficient indexing techniques. Without such techniques query processing would simply take too much time. Considering a possibility based database modelling approach, imperfect data are modelled using a possibility distribution. The possibility distributions used for data modelling in different database records have to be indexed in order to support a faster processing of query conditions that act on the imperfect data. In this paper we study the indexing of imperfect temporal data in the Diplomata Belgica database, which has been co-developed by our research group. Diplomata Belgica is a relational database describing medieval charters written and issued in the southern Low Countries. More specifically, we study how imperfect data on the issuing date of a charter can be modelled and indexed in order to support searches for charters with an issuing date that is compatible with `fuzzy' query preferences provided by the user. A novel, so-called Interval B ⁺ -Tree (IBPT) indexing technique is proposed and some illustrative examples of (the handling of) complex, realistic queries are given.