Jared Largo says he just wanted to spread cheer before St. Patrick's Day, which is celebrated Wednesday.
His supervisors had video that showed him using the stamp. Says Largo: "The one that got me in trouble was the one by the time clock."
He says he planned to remove the shamrocks eventually, according to The New York Times.
Largo says a Department of Transportation judge gave him 16 days of unpaid suspension. The DOT says it cannot comment on personnel matters.
Largo is trying to enjoy the extra free time. He'll attend the Manhattan parade on Wednesday.
Says Largo, who's half-Irish: "I was thinking of renting a green tux."