We consider the inverse time problem for the non-linear heat equation in the form u(t) + Au = f(u), u(1) = X, where A is any non-negative. self adjoint operator. Using the strongly continuous contraction semi-group generated by A(beta) = -A(I + betaA)-1, beta > 0, we derive an estimation of the error on the whole interval (0,1] between u(t), solution of the initial problem, and upsilon(beta)(t), solution of the regularized problem: upsilon(beta)(t) + A(beta)upsilon3(t) = e-(1-t)betaAAbeta.f(upsilon(beta)), upsilon(beta)(1) = X.