"Cairteal an dèidh ceithir."
Translation:Quarter past four.
An-dè, an-diugh etc. are adverbials of time. The an- often corresponds to to- in English or the in Scots.
But an dèidh isn't an adverbial of time. It isn't even an adverb. It is a compound preposition. We will learn about these later, but they correspond to phrases in English such as at the side of, in front of. In this case it its thought to originate as in [the] end [of]. So an is a preposition meaning 'in', and dèidh is thought to have meant 'end'. Two words, with no need for a hyphen.
It now turns out that even if it were an adverbial of time, there is inconsistency about whether the hyphen is required. See https://forum.duolingo.com/comment/36403042?comment_id=39609128