or I suppose you could ask on a Q&A site, where you'd be told:
1940 - Churchill became Prime Minister May 10, but only met in the War Rooms occasionally until bombs damaged No. 10 Downing Street Oct. 15. After that, he would meet in the War Rooms on a regular basis. All clocks in the rooms today are set a 5 pm, when the War Cabinet met on Oct. 15.