According to the Fifth Doctor, Planck time was the time it took for light to travel across a defined distance, whereas interstitial time was so infinitesimally small that it existed between the very beats of Planck time.
As a result of being smaller to Planck time, interstitial time could not generally be measured. However, Professor Kalu claimed to have been able to measure interstitial time by isolating a chronon seed. (AUDIO: Interstitial)
Behind the scenes Edit
In the real world, Planck time is more specifically the time it takes for light to travel in a vacuum across what is known as a Planck length. Like the Planck constant, mentioned in PROSE: The Secret in Vault 13, both units are named for the physicist who proposed them, Max Planck.