Actually, you could just say. "No, that's not right, it was only a year ago." (more formally, you could also say, "No, I'm mistaken, it was only a year ago." We have a verb "misspeak" == to say something that is not correct, by mistake. "No, I mispoke, it was only a year ago." I misspoke when I said that Mr. X did "Y" (I was mistaken).

"I stuffed up in what I said" is a very casual/informal way of saying you made a mistake in what you said. Rena's suggestion "I misspoke" is a polite way of saying the same thing.

"It was a slip of the tongue: it was one year ago."

I don't think there is one. Usually we would just say something like “I went there two years ago. Sorry, it was one year ago.” Rena's 'misspoke' is probably correct, but I don't think it is very common.