(Twilight momentarily stops when she pulls out a history book.)

...A friend of mine is also on that list. Back in December, he changed his name to Andrew, but the file lists him as Handy. It also listed when Nightmare Moon possessed me, which was October of last year. Not all of those individuals were around at that time, but if she can see into others' universes, then she's probably studied them while they were still home.

If I had to guess, I'd say it began before we arrived in Equestria and continued from that point onwards.