Hiya. This is me doing the dance of Yay! Liz is Writing Something!

And, hrm. Nope, no clue. We've got that one flashback where Fraser finds Dief in the hole, so you can extrapolate from how old Fraser looks at the time and what you know of dog development, but no exact date.

Which is kinda disappointing because it would be nice to have a canonical answer, but is also nice to have blank, for Authorial Purposes. ;)