In response to your question, if I could just mention 2018 and the timing, the reason why the OAS and the international community is calling for early elections is that the constitutionally permitted recall referendum—there's a process to recall the president gathering a certain number of votes—was blocked by Maduro; he refused to allow that to proceed. As a result, he unconstitutionally stopped a process that was created by the constitution when a certain number of signatures are gathered, and he refused to honour that signature gathering effort. This is why the international community is not focused on the date of the expiration of his term now, but is calling for early elections in 2017. It's because he has already unconstitutionally blocked the recall referendum, which would have been the right or the first best alternative to have him be gone.
On May 16th, 2017. See this statement in context.