Mitsuki is using Aya’s last name, so this is set at some point before.
When did I miss that part? What was the reason? Stage name? Unofficial yuri marriage? I do see they have non matching rings.
Oh, I didn't mean either of them changed their own names. Before the time skip they address each other as Oosawa-san and Koga-san, and after the time skip they've switched to Aya-san and Mitsuki. You can use that to tell when the volume 4 bonuses are placed.