If you say that something has been happening since time immemorial or from time immemorial, you are emphasizing that it has been happening for many centuries.
例句
It has remained virtually unchanged since time immemorial...實際上,自古以來它幾乎沒有任何改變。From time immemorial there has been the belief that there are good and bad places to be.人生既有順境，也有逆境，這種觀念自古有之。
ADJ
無法追憶的;遠古的;古老的
You use immemorial to indicate that something has existed for many centuries.
例句
...a modern version of an immemorial myth.遠古神話的現代版本
英漢詞典釋義
adj.
無法追憶的,遠古的
The Barton family have lived in this village since time immemorial.巴頓家族自古以來就住在這個村里。
英英詞典釋義
Adjective
1. long past; beyond the limits of memory or tradition or recorded history;