  A spatially-independent model that can be used to calculate the temporal
evolution of the electron/ positron spectrum in a spherically expanding pulsar
wind nebula is presented. The model is applied to the young nebula G21.5-0.9,
as well as to the unidentified TeV sources HESS J1427-608 and HESS J1507-622.
The parameters derived from the model strengthens the idea that the
unidentified sources can be identified as evolved pulsar wind nebulae.
