Jim's right: you mixed the military stile (quinze heures) with the colloquial style (et quart). You can either say:

"quinze heures quinze" (military style 24 hour clock) or "trois heures et quart" (colloquial 12 hour clock). With the 12 hour style, you can also add "du matin" (a.m.) or "du soir" (p.m.) to remove any lingering ambiguity.

Hi Beatriz,

Copied from the lesson:-

"When the time expressed uses hour numbers above 12 (in the "24-hour clock" -> 13h, 14h...), you use instead quinze, trente, quarante-cinq, probably for pronunciation (and elegance) reasons."

eg.    "Il est seize heures quinze. It is quarter past four PM."

I suspect this is the reason why your answer was considered incorrect (after mid-day)

Hope this helps.

Jim