"The Power of Three" takes place in 2016-2018? I don't recall the dates being mentioned, and I can't find the mentions in any wikis. Oh well, so what if Clara is from the 2020s? It's not as though it's unusual for the Doctor to have companions from other times, and the 2020s aren't particularly futuristic. Hell, Sarah Jane was supposed to be from the 1980s during the Third/Fourth Doctor's era, wasn't she?