Everyone in Beantown knows the clock is ticking — and rather loudly — which makes the next two seasons so critical. The Globe explains: “It was no coincidence that Celtics president Danny Ainge signed three free agents this summer to two-year contracts. The window for success with this current crew has been established, and June 2012 is the deadline. That’s when the mammoth contract of Kevin Garnett comes off the Celtics’ books, a cap relief of more than $21 million, leaving the deals of Paul Pierce, Rajon Rondo, and Avery Bradley as the only ones remaining on the payroll. Then it will be time to rebuild. But for now, the Celtics might as well have ’2012’ plastered over the locker room door — like the ‘Play like a Champion Today’ sign the Notre Dame football team has. Each player should reach up and slap it for motivation, because that’s when the era of the Big Three likely will end. And nobody in the organization denies it. Ainge’s maneuvering will give the Celtics nearly $30 million in salary cap space in the summer of 2012, when the new collective bargaining agreement will be ratified and owners are expected to have more control over guaranteed contracts and revenue sharing. But enough of the futuristic talk. The time is now, and Ainge has no problem relaying that message to his team. ‘Well, we’ll enjoy the two seasons while we have them,’ Ainge said. ‘We’ll take it one season at a time. But right now, we’re doing all we can to win a championship right now. Our focus is just this year and we’ll worry about the following year next summer. I don’t think it’s added pressure. I think those guys know where they are in their careers. They know their window is short. They all know what opportunity lies ahead and there’s not a lot left for them.’”