I have just used my 90/180 day visa which was purchased at the beginning of April on my arrival to Dalaman 5th August. I did ask the man who showed passengers to the visa desk if I needed to pay, just to check. He thought for a moment, working something out in his head reeling the months of the year out, then said no go pay visa. Having read the link about visa's and using the calculator my visa was in date for use. So I asked again at the visa desk, stating 90/180 visa. I was asked how long my stay was, which fell inside the 180th day. With this, he told me to go to passport control. My visa from April was stamped, with no further payment. So it worked for me!