"...just as the human mind cannot comprehend time, so it cannot comprehend the damage that will ensue if we presume to tamper with its laws."
-- Professor Saul Croaker (Pm)
Professor Saul Croaker was an “Unspeakable” who specialised in time-magic (Pm).
He developed Professor Croaker’s Law, which states that “the furthest someone can go back in time without the possibility of serious harm to the traveller or time itself is five hours” (CC2.16).
Croaker was obsessed with the dangers of Time Travel, and strictly guarded the use of Ministry Time-Turners.
Saul comes from the Hebrew name שָׁאוּל (Sha'ul) which means "asked for, prayed for" (Behind the Name).
Croaker is "something that makes a croaking sound". It can also be a type of fish or a slang word for a doctor (Merriam Webster).