How do you say, in French?

"un an plus tard" is correct.

"tard" is never used by itself. It is always preceded by the word "plus" as in "plus tard." "Later" in French is "plus tard."

Degrees of comparison

http://www.laits.utexas.edu/tex/gr/adj6.html

Tard = late

plus tard = later

aussi tard que = as late as

lettuce = l. a. salade milk = le lait ice cream = l. a. glace bread = le ache banana = l. a. banane orange = l'orange apple = le pomme carrot = le carrot cake = le gateau egg = l'oeuf cheese = le fromage ear = ? eye = ? red = violet crimson = rose orange = orange yellow = jaune black = noir pen = le stylo pencil = le crayon table = le bureau frog = ? horse = le cheveau get dressed = ? blouse = ? pants = pantalon shawl = ? teach = gare lady = ? woman = ? child = ? guy = ? Boy = le garcon plate = plat cup = le verre soz if incorrect spelling