We consider the regularization of the inverse time problem for the nonlinear heat equation in the form , , where A is any non-negative, self-adjoint operator. We assume that the solution u(t) belongs to and the function f is locally Lipschitzian. The regularization is made through an integral equation associated with a strongly continuous semi-group. We derive an estimation of the error on the whole interval (0,1]. Numerical results are given.