Om is the preposition used for talking about distance in time to a future event. In this case the other prepositions have other meanings.
- De betalar i ett halvår = They pay for half a year
- De betalar på ett halvår (colloquial) = They pay (off the loan) during half a year
- De betalar om ett halvår = They will pay half a year from now.
Alan, the others here are asking questions rather than making points, wouldn't you say?
And no, it does NOT require further context to correctly translate the Swedish DL gives us here. What is required is an understanding of what the various Swedish prepositions mean and how they are correctly used.