  Utilizing the bounds on primordial magnetic fields (PMFs), their
contributions to secondary gravitational waves (GWs) and the results from the
pulsar timing arrays (PTAs), we arrive at constraints on the epoch of
reheating. We find that the combined spectral density of primary and secondary
GWs (generated by the PMFs) can, in general, be described as a broken power law
with five different indices. We show that the PMFs that have a blue tilt and
satisfy the other observational constraints can generate secondary GWs of
strengths suggested by the PTA data.
