Time immemorial is time extending beyond the reach of memory, record, or tradition. The implication is that the subject referred to is, or can be regarded as, indefinitely ancient.

The term has been formally defined for some purposes.

• In English law, time immemorial means "a time before legal history, and beyond legal memory." In 1276, this time was fixed by statute as the year 1189, the beginning of the reign of King Richard I. 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. Source: The public domain Webster's Revised Unabridged Dictionary (1913)
• The Court of Chivalry is said to have defined the period before 1066 as "time immemorial" for the purposes of heraldry.

Compare against Prehistoric.

References

• Reference for the heraldry date
• Reference for the heraldry usage

Time | English law