Validity of a 30 day China visa

It means that you have to enter China anytime within the validity, but once you enter China, you can stay in China not more than 30 days after the date of entry.

For example, your visa is issued on March 1, 2012, the last date for you to enter China is June 1, 2012, you can enter China at any date from March 1, 2012 to June 1, 2012. Once you enter, you can stay in China for 30 days at most counted from the date of entry if your visa states "Duration of Each Stay 030 Days After Entry"