In Dirac's 'Classical theory of radiating electrons', submitted in 1938, the electromagnetic radiated field is defined as:
$$F_\text{rad}^{\mu\nu}=F_\text{ret}^{\mu\nu}-F_\text{adv}^{\mu\nu}$$
Where $F_\text{ret}^{\mu\nu}$ is the electromagnetic tensor for the retarded field, which can be obtained from the Lienard-Wiechert potentials, and $F_\text{adv}^{\mu\nu}$ is the electromagnetic field for the advanced field, in which the charge effect propagates backwards in time.
I wonder which is the meaning of this definition. If an electron accelerates at a given time, the effect of that acceleration will propagate in two different regions of the space for the retarded and the advanced field (in the forward and backwards light cone). Also, for a given point $P$ we can have $F_\text{rad}^{\mu\nu}\neq 0$ even if the particle has accelerated neither at the retarded time $t_\text{ret}$ nor at the advanced time $t_\text{adv}$ but it has accelerated between $t_\text{ret}$ and $t_\text{adv}$.