The city is a complex object and its descriptions by urban actors have evolved over time. Therefore many ways to investigate it have emerged. In this paper, we propose to study cities through the combination of units of analysis, called “urban entity” and defined by a function, a form and a date on a continuous timeline. However, while the dating of items is often done in absolute time, the way it is determined is not always clearly formalized by archaeologists and historians. We present a formalization of the dates of urban entities, which take into account certainty and estimation. Each entity is defined by numeric dates so it is possible to explore the statistical distribution of urban entities through function and chronological periods, based here on 50 years (units). Then, the analysis and comparison of change and sustainability of three towns from the 1st to the 18th century may be systematized.