McKendrick partial differential equations in demography provide the state of a population depending on chronological time T and age D, and involve two time evolutions: the chronological one t is an element of [0, T] bar right arrow t >= 0 and the calendar age t is an element of [T - D, T] bar right arrow t - (T - D) >= 0, both with constant velocity equal to 1. The calendar age evolution chaperons, so to speak, the evolution t is an element of [T - D, T] bar right arrow x(t) of the state of a system. Some physical, biological, and economic problems motivate the introduction of variable durations with variable velocities (representing the fluidity of time) offering mathematical metaphors of a "subjective fleeting specious time" passing more or less slowly. Variable durations are no longer prescribed, but chosen among those available and regulated: the joint evolution of the variable evolution and the state is assumed to be governed by a differential inclusion (or control system) and provides, as a byproduct, the unknown temporal windows on which they evolve together. In economics, for instance, the state is a commodity, its velocity a transaction. If a cost function is defined on the fluidity of the variable evolution and the transaction of commodities, their cumulated cost over the fluidity-transaction pairs could be minimized. Slowing down the fluidity, which widens the investment period (from the milliseconds of high-frequency markets to the centuries of cathedral building) by inventing a shareholder value tax and decelerating transactions (by implementing the Tobin tax) could be an objective of salubrious financial and corporate management. This was the real motivation of this study, which finds applications in other fields where duration, this particular meaning of time, should have a value.