At the entrance to a big office in London, there was a piece of paper which all men had to write names on when they arrived each morning. At nine o'clock, a young woman who lived in a small room above the office, had to draw a red line under the last name, and anyone who was coming after that had to explain (说明原因) to the manager. Whenever there was a thick fog in the city the first person to arrive late usually wrote the reason 'Delayed (延误) by fog' and then everyone else after him then just put 'ditto' below. One morning the first man who arrived late wrote 'My wife has given birth to a baby this morning' instead of 'Delayed by fog.' Twenty or thirty people after him put 'ditto' below as usual. The office hour started ______ .