Fractional calculus is applied to analyze and model the viscoelastic properties of different elastomeric materials. We find that the order of the fractional time derivative governing viscoelasticity is material dependent. Model validation uses experimental data describing viscoelasticity of the dielectric elastomer Very High Bond (VHB) 4910 and 4949. Since these materials are known for their broad applications in electromechanical smart structures and robotic systems, it is important to characterize and accurately predict their behavior across a large range of time scales. Whereas integer order viscoelastic models can yield reasonable agreement with data, the model parameters often lack robustness in prediction at different deformation rates. We find that fractional order models of viscoelasticity provide significantly more accurate predictions of rate-dependent nonlinear deformation. Prior research that has considered fractional order viscoelasticity often lacks experimental validation and contains limited links between viscoelastic theory and fractional order derivatives. We therefore use fractional order operators to experimentally validate fractional and non-fractional viscoelastic models in elastomeric solids and quantify the uncertainty of the fractional parameters using Bayesian statistics. Upon calibration at a single deformation rate, the fractional order model is shown to more accurately predict viscoelastic stress over deformation rates spanning four orders of magnitude.