The clock change to Greenwich Mean Time, in October last year, led to 71 emergency calls disappearing from London Ambulance Service control room screens.
As the news emerged today, the service confirmed that the calls had gone missing from a range of displays running on its in-house CTAK system.
CTAK was first installed in 1992. But London Ambulance Service attempted to replace the system last June, with the CommandPoint software from supplier Northrop Grumman. The failed integration of the new system led to a system crash, and London Ambulance immediately reverted to CTAK.
London Ambulance said the 71 calls only disappeared on some screens, but a number were answered late. It said none of the missing calls had been about a patient in a life-threatening condition, but conceded that "a few" callers would have benefited from an earlier response.
The problem is likely to raise concerns that calls could go missing from screens again, during the next clock change in four weeks' time.
London Ambulance Service told Computerworld UK that it still planned to implement CommandPoint, and may do so in the coming weeks. The troubled launch last June prompted a six month review in which the service called for a better system and new specifications.
A spokesperson for the ambulance service said that the issue on CTAK last year was "immediately identified by staff, and after 25 minutes was resolved by our technical team, who were able to account for all the 999 calls in the system".
"A clinical review of the calls that hadn't been visible found that no patients were in a life-threatening condition, but a few may have benefited from either an earlier response or a ring back to make a further assessment."