The structure of a calendar can be described by a sequence of quasi-affine forms. Such a sequence, that will be called quasi-affine basis, generalizes the notion of numeration basis. The problem of the date conversion is therefore reduced to the writing of the Julian Day in such a basis. A discrete straight line recognizing algorithm enables us to obtain the good quasi-affine basis. As examples are given the julian, gregorian, islamic and judaic calendars.
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