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In law, it means that a property or benefit has been enjoyed for so long that its owner does not have to prove how they came to own it. In English law and its derivatives, "time immemorial" means the same as "time out of mind", "a time before legal history and beyond legal memory". In 1275, by the first Statute of Westminster, the time of memory was limited to the reign of King Richard I, beginning 6 July 1189, the date of the king's accession. Since that date, proof of unbroken possession or use of any right made it unnecessary to establish the original grant under certain circumstances.
As Bryan A. Garner notes:
“Memory of man runneth not to the contrary This immemorial phrase expresses immemoriality – or the point before which legal memory began (fixed as the Year 1189), also known as time immemorial ... By the early 16th Century, English courts used it to restrict the use of custom ... The phrase is frequently used in an extended sense in American judicial opinions as well as in legal commentary.”
The Prescription Act 1832, which noted that the full expression was "time immemorial, or time whereof the memory of man runneth not to the contrary", replaced the common law burden of proving "time immemorial" for the enjoyment of particular land rights with statutory fixed time periods of up to 60 years.