Analytical solutions in well-test analysis are oriented towards fluids with a constant viscosity and compressibility in a porous medium with a constant porosity. To use these solutions in gas-flow situations, one needs to apply the pseudo-pressure and pseudo-time transformations. Thus, the non-linear diffusivity equation for gas flow is transformed into a linear one, allowing one to use the solutions for fluids with constant compressibility, viscosity, and porosity. Although the computation of pseudo-pressure is reasonably accurate, the conventional computation of pseudo-time by direct integration of the compressibility-viscosity function over time can result in significant errors when modelling gas reservoirs with residual fluid saturation, rock compressibility, and a large degree of depletion. Errors in calculating the pseudo-time result in substantial errors in the material balance, which has an adverse impact on reservoir modelling and production forecasting. In this study, a new method for computing pseudo-time is presented. This method is based on the material balance equation that considers the rock and fluid compressibility. This formulation honours the material balance equation in all situations. Examples are presented to show that the problems with computing pseudo-time, using the traditional definition, can be resolved when the new method is used. Accurate computations of pseudo-time allow one to use the solutions for fluids with constant compressibility and viscosity for modelling and forecasting gas production.