Is the 3 month validity until the start of the trip or the end

I would like to apply for a visa in my current country but intend to travel for 2 1/2 months before I arrive in China. Does the 3 month validity period just mean that I have to enter the country within 3 months of getting the visa or does the entire trip have to happen within this 3 month period. So theoretically I could enter China 89 days after I got the visa and stay for a month?