What is time at large?

It comes from the prevention principle. An owner cannot hold a contractor to a date that the owner's own act made impossible.

When time is at large, the owner loses the right to liquidated damages for late completion. A court or arbitrator decides what a reasonable time was.

It is rare in Australia. Most contracts have an extension of time clause that covers owner delay. The clause moves the date for owner delay, so the contractor still has a date to meet.

Treat time at large as a last argument. The strong path is the extension of time clause. Give notice on time and claim the days.