Today, I got the reply from theymos that he took over the forum since 2012.
Here is a detailed clarification from theymos, and I thought that @xtraelv should add it into the OP, because it is one of important things of the forum.
There was a period where both Sirius and I were active administrators. For convenience, I reckon Jan 1, 2012 as when I became head administrator, though it was actually a gradual transition starting before and ending after that.
I've added the whole quote to the introduction rather than adding it to the timeline.