When to apply to renew?

I am in Xinjiang. I was given an L visa for 30 days, I came in on May 17 but my return flight is July 1st. I want to apply for an extension as I know I will go passed my 30 day visit but since I will be traveling all over Xinjiang and have no idea when I'll be back in Urumqi, I want to get my extension tomorrow. However if I do that, adding another 30 days on top of May 22nd only lets me stay till June 21st. What should I do so that I can return home safely on July 1st?