Hi Terpe!
I arrived in UK on a student visa before I met my husband. I got my spousal visa after we got married and I checked that the expiration date of my visa is on June 5, 2014. I do believe that I fall on the 28 days before the 24 month anniversary. I was just worried that booking on those times for a slot might be hard like when booking to apply for spousal visa. But I guess there is no choice but to wait for those time frames.
Thank you!