je

Je is the escape hatch: it is the one preposition with no fixed meaning, kept in the language for the cases where no other preposition quite fits. It carries clock times, measurements, and a handful of fixed turns like kredi je and teni je la mano. Where je would be vague you may instead use the accusative, so je la tria horo and la trian horon both mean at three. Use it sparingly: if a real preposition fits, that one is better.