In 2 weeks – i.e. in 14 days from now. Within 2 weeks – i.e. during today or the next 14 days, not later. But what about ” within 100-200 days”? I would say “from now and not later than in 100-200 days”. But a native speaker told me that actually it means “from 101th to 200th day”, i.e. not from now till the upper limit.
