Herbby is right, I doubled checked my 30 day extensions that I purchased at immigration and the new 30 days starts on the day you receive that visa. So if your current one expires on April 2 and you go to immigration for an extension on March 31, your new 30 days will begin on March 31. For your second question, all Thai immigration cares about is that you don't overstay your visa. Each day of overstay is about 500 baht a day I think.