Does "validity" of your visa only apply to your entry date?

Yes, the validity of China visa only apply to the entry date. If you enter 1 day before the expiration date, you can still stay for the full 30 days of your "duration of stay".

For example, your visa is issued on March 1, 2012, the date for enter before is June 1, 2012, it permits 030 days duration of stay after entry, you enter China on May 31, 2012, you can stay in China for 30 days counted from the date of entry.