The need for knowledge of time-dependent viscoelastic material functions has been growing with the increased use of an accurate engineering methods for rigorous predictions of the plant materials behaviour, such as the finite element method (FEM) and the boundary element method (BEM). Essentially, only linear viscoelasticity is considered for which the correspondence principle applies. A new method for computing the time-dependent Poisson's ratio of linear viscoelastic materials, using discrete time-measurements of the uniaxial creep compliance of unconfined and a laterally constrained cylindrical specimens of the material obtained in double creep experiment, is developed on the basis of the constitutive convolution integral equations. The approach proposed solves the problem in Laplace transform domain and relies on numerical inversion for the determination for the time-dependent Poisson's ratio. The method combines effectiveness and accuracy and is general enough to cover both viscoelastic solids and liquids.