User blog comment:GianG/Merlin characters ages/@comment-5674726-20141116000448/@comment-5102537-20141116165436

GianG wrote

"Gwen didn't say: "it has been a year since Arthur discovered Morgana betrayed him", but she said " it has been a year since Morgana betrayed Arthur". " 

Gwen's exact words are: "It's been a year since Morgana betrayed him."

Meaning "since Morgana betrayed Uther". "The Darkest Hour" is the first episode of season four and Morgana's open betrayal happened in the last episode of season three ("The Coming of Arthur"). A year has 12 months and Gwen says "a year", so a year has passed between season three and season four.