Surely the JCT D&B SFC includes mechanism for extending time due to reasons attributable to Employer => in what circumstances would time be at large?
Very unlikely that there would ever be a time at large scenario because that would leave the Contractor to undertake and complete the Works whenever it suited him/her. At best there could be a situation where the Contractor, rather than having a specified date by which to complete; had a reasonable period of time in which to complete. That is hugely different to time at large.