Wiktionary
year dot
n. (context idiomatic British Ireland English) A very long time ago, from the beginning or as far back as one can remember.
WordNet
year dot
n. as long ago as anyone can remember; "he has been a conductor since the year dot"
Usage examples of "year dot".
Furthermore, he is confined to an ancient power-driven wheelchair and by skillful chemistry derives a frugal living from the soil and one of the ancient B-type converters dating back to the year dot.
But as I've said, it's been Francezci family property, oh, since the year dot.