The validity of the visa starts when the embassy issue it on your passport, once the tourist visa is issued to you, you can enter China by using it, the stay duration of visa starts when you get it stamped on arrival. The stay duration is counted from the date of your entry. I don't know whether the 30 days tourist visa means 30 days stay duration after entry or 30 days validity on your visa, you'd better tell me more information about your visa.