Even though I have never watched Dr. Who, I can tell it's a reference from that show (hopefully!). I am not an artist whatsoever, so many things amaze me when it comes to drawings. But this is great. I especially love how shocked (especially Time Turner's "What is this-I don't even" face) everyone looks except Derpy, with her happy-go-lucky attitude. This picture might put me into a position to watch it if my Whovian friend don't force me to first. It is very realistic and well done, and I can tell tell you take pride in your work. Keep drawing!
What I like the most is that the current Mayor of Ponyville used to be an assistant of The Doctor. I've seen plenty of "other" assistants in different pictures and fanfction: Derpy, Twilight, Trixie... but the current Mayor as an assistant of a previous Doctor? That is just SO original. And it FITS!