  The interevent time distribution characterizes the temporal occurrence in
seismic catalogs. Universal scaling properties of this distribution have been
evidenced for entire catalogs and seismic sequences. Recently, these universal
features have been questioned and some criticisms have been raised. We
investigate the existence of universal scaling properties by analyzing a
Californian catalog and by means of numerical simulations of an epidemic-type
model. We show that the interevent time distribution exhibits a universal
behavior over the entire temporal range if four characteristic times are taken
into account. The above analysis allows us to identify the scaling form leading
to universal behavior and explains the observed deviations. Furthermore, it
provides a tool to identify the dependence on the mainshock magnitude of the c
parameter that fixes the onset of the power law decay in the Omori law.
