nekoć

1. Croat. (about the distant past) once; formerly; long ago
   Na tom mestu nekoć je stajala crkva. — A church once stood on that spot.
   Ovaj grad je nekoć bio važna luka. — This town was once an important port.

nekoć is characteristic of Croatian (western-variant) usage and is rare in Serbia, where it is perceived as Croatian. In Serbia: nekad or nekada.