The property of being isochronal.
"The science of timing lies in achieving isochronism of the regulating organ of a timekeeping instrument. The principal factors that can impair a balance's isochronism are the escapement, incorrect poising of the balance and spring, play between the curb pins and the balance spring, centrifugal force and magnetic fields." (FHH)
Isochronous
"Occurring in equal periods of time. Horologists say that the oscillations of a pendulum or balance are isochronous when their duration is independent of their amplitude." (Berner)