Today, I learned that ‘om’ is the prefered preposition to talk about distance in time to a future event. The exercise I did used a sentence ‘De betalar om ett halvår’. I would have used ‘De betalar på ett halvår’ or ‘De betalar i ett halvår’. I saw the comments on the post here, but am not convinced that ‘De betalar i ett halvår’ could be taken to construe ‘They pay for half a year’. Shouldn’t that be ‘De betalar för ett halvår’ instead? I’ll try and ask around!