Since Time Immemorial

Merriam Webster: a time antedating a period legally fixed as the basis for a custom or right.

While listening to the BBC, I was amazed to learn that this phrase has a precise meaning in English Law.

Checking online, I was even more astonished to learn just how long ago this phrase was defined. Wikipedia:

In 1276, this time was fixed by statute as the 3rd September 1189, the date of the coronation of King Richard I (Richard the Lionheart). Proof of unbroken possession or use of any right since that date made it unnecessary to establish the original grant. In 1832, the plan of dating legal memory from a fixed time was abandoned; instead, it was held that rights which had been enjoyed for twenty years (or as against the Crown thirty years) should not be impeached merely by proving that they had not been enjoyed before.