Shenzhen Visas

Hi! I'm British with a UK passport, and am staying with my family in Hong Kong from December 2015 to February 2016. We're travelling into China twice, and I have to give the dates and destinations of these two trips. However, I'd also like to go on some day trips to Shenzhen for which, in the past, I have bought single entry permits at the border. It seems sensible to apply for a 6 month multi-entry tourist type, but how do I get round the fact I can't show the dates for the Shenzhen visits (I probably won't know myself until the night before) on the application form? Thanks for your thoughts - John.