I have a visa valid for 30 days. I will arrive on 28 July and leave on 28 Auguest.

Qaithsi, I'm afraid it will be considered as overstaying.
But it's okay if you arrive on 29 July and leave on 28 August because the duration time in China will start from 0:00 of the next day after your entry.

Dear Lisa,

Thank you for your reply. I will reach at 11 pm. If I wait till midnight for change of date before approaching immigration, can I get an extra day? Thanks.

If you reach China at 11 pm on July 29, the 30 days will start from July 30, and then you can stay till August 28. In this case, you actually stay for an extra day in China.

Hi Lisa, No the actual scenario is like this: I will reach at 11 pm on 28 July and return on 28 August, which means if they start counting from 29 July, it will be 31 days. What I was suggesting is that if I do not exit the airport right after landing and spend one hour inside so that it becomes 29 July when I reach immigration counter. Let's say my passport is stamped at 00:10 am on 29 July. In this case will they start counting from 30 July and I will be able to fly back on 28 August? It's expensive to change tickets, so if this options can work that will be great.
Thaks for your help.

Dear, you can have a try, but I cannot assure if it can work. Or there is an alternative way. You can extend your visa 7 days prior to its expiry date and it won’t cost a lot. In this case, there won’t be any problem.