Right, but don't worry about it. It was your birthday, so we'll give you some slack.
There are actually
three pronounciations: Teatime, or as Susan Sto-Helit(sp?), AKA the granddaughter (don't ask who her grandfather is, it's complicated), calls it, "4 o'clock"; "Teh-ah-tim-eh" in the novel; and "Teh-ah-tie-mee" in the live-action version.