Are exact dates required for a double entry visa to China?

When I apply for the visa do I need to state the exact date of the second entry? For example, I plan to enter China on May 1 (first entry), then tour the country for several days. Then at some unknown date I want to leave China to visit North Vietnam for a few days, then I want to reenter China to finish my China trip, and finally return to the USA. Do I have to know in advance when I aply for the visa exactly what date I will reenter China from N. Vietnam?