"It was in November 2010," the Blues boss said on Tuesday, without drawing breath.
The fact Di Matteo could remember the date at all - he was not even at the club then - was impressive.
But the statistic was also an alarming reminder of how far and fast Chelsea allowed themselves to fall in the Premier League during the past two seasons, despite their astonishing Champions League triumph.
Becoming European champions has given them an opportunity to lay that ghost to rest tonight when they play their first home game of the season against Reading.
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The match has been brought forward from the first weekend in September due to Chelsea's involvement in the UEFA Super Cup a week on Friday.
That means a win or a draw would see them go top of the Premier League and Di Matteo added: "We haven't been top for a long, long time.
"We have the opportunity. Hopefully, we will play a good game and manage to win."
All eyes will be on Eden Hazard following his dream Premier League debut at Wigan three days ago.
Such was the £32million man's impact that he is already being tipped to be one of the stars of this season and beyond.
Di Matteo warned against making snap judgments about the 21-year-old playmaker but admitted he could see similarities with Gianfranco Zola, who he starred alongside for both Chelsea and Italy during the 1990s.
He said: "It's difficult to compare players. Gianfranco was a wonderful talent, an artist I would say.
"Eden might become an artist as well. After one game, it's probably a bit too early.
"He had a good start, but it's one game out of 38 in the league.
"It certainly gives him and the others confidence, starting with a new club like that. It was a pleasant day for him.
"He's a talented man, a talented young boy. We'll have to see how he grows into a man, and how quickly he will be an important player for Chelsea."
Di Matteo was right to play down comparisons between someone who has made two appearances for Chelsea and a man often voted the club's best player of all time.
But an assist less than two minutes into your Premier League debut and another five minutes later certainly puts you on the right path towards greatness.
Di Matteo declared there was even more to come.
"It was great to see him enjoying himself as well, and getting his teeth into the Premier League," he said.
"You're never quite sure how the new players will adapt, and I'm sure it will take him a bit of time to get used to the physicality and tempo of the Premier League."