Throughout the Doctor Who verse, one thing I have noticed is The Doctor using "Earth Time". When going through time, he will often say the year that the Earth is at, rather some sort of "Central Universe Time". Is there any explanation for this, or is it simply to make things easier for his assistants to understand?
As with all speech, I would assume that the Tardis' Translation Circuit maps the year the doctor says to something that we can understand. As the Doctor said it's -
... gift of the TARDIS, a telepathic field that gets inside your brain — translates.
The End of the World
It would make sense for it to translate it into a number or system that was understandable.
Tho in conversation with his assistant, it would make sense for The Doctor to keep things in "Earth Time" as I imagine having to answer the question "but what year is that really?" would be tedious.
The 3rd doctor was sentenced to spend time on Earth and had his TARDIS taken away (forcing him to use a transport called Betty).
So he spent fighting aliens on Earth until he got his Type-40 TARDIS. After all that he has studied Earth and learned many things, including to use Earth-time.