The 31-year-old's current deal runs out in 2009 but if he reaches a specified number of appearances he automatically wins himself an extra year.
Hartley, who is in the Scotland squad for Friday's friendly against the Czech Republic in Prague, would not reveal exactly how many games he had to play to trigger the clause in his contract, but he said: "I want to stay as long as possible at Celtic.
"I've got a year left with an option of another year if I play a certain amount of games.
"I have to do that and show that I am good enough. That is the main aim.
"I might not play every week, which happened this season, but I will get on with the job and try and do the best that I can.
"But I won't worry too much about these things.
"If it happens, it happens; if it doesn't, it doesn't."