Aye, I guess I meant very very "approximate" not "exact"
So what we need is a fixed observer point for all Time Travel operations. Given we already use Greenwich for our Earth based coordinate and time systems, I propose we erect a huge needle in the middle of Greenwich. This needle (the "Time Needle" perhaps) will be our (0,0,0,2012-10-02) reference point.
We'll have it pointing as straight up as possible and that can be the y axis.