This paper demonstrates how the extended duration calculus [4] can be used to support the approach to documentation of computer systems presented by in [1]. This approach uses the general concept of mathematical relations to specify properties, while the calculus of durations provides the means to reason about such specifications, and in particular, prove formally that a design implies the requirements. The presentation is based on an example originally presented in [2], and later reformulated in [3] following the approach described on [1]. In the present paper we introduce all needed relations, express them in terms of duration calculus, and formally verify software design acceptability.