Abstract. The paper reports on a study to develop solutions for a chosen problem in two related, but di erent languages. Moreover, the languages re ect two related, but di erent programming paradigms: logic programing, and constraint logic programming, respectively. We use Prolog to describe calendars and their mutual conversions. Next, we use Prolog III to describe the same. We discuss suitability of both languages for this kind of task. Prolog III as a logic programming language with constraints allows writing a program which is both more general (i.e., covering a broader range of cases) and more abstract (i.e., expressed on a higher level of abstraction due to the use of constraints).