Just back from Calis and had an interesting experience at Dalaman coming in. The travelagency cancelled our flight to Turkey and offered a flight two days earlier. It never crossed my mind to change the date on my visa. At the passportcounter the grumpy old man looked at the passport, zipped it through a machine, looked at it again and wanted to see the printed visa. Then he pointed out my visa was not yet valid... and stamped the passport. Upon leaving at Dalaman Disco also no problem.