The year dot
A very long time ago; too long ago to be dated.
This is a fairly recent phrase which originated in England at the end of the 19th century. It is still in use there, although less so elsewhere. The earliest record I can find of it in print is from Lands of the slave and the free: or, Cuba, The United States, and Canada, by Henry Anthony Murray 1857:
"But as our ancestors adopted this system 'in the year dot, before one was invented..."
See also - jot or tittle.