If cell blank than adjacent cell blank (with drop down)

Hi,
I'm trying to create an office calendar. B7 contains a drop down list of reasons for out of office (personal time, sick day, etc.) If B7 is blank I want C7 and D7 (where the in/out times normally would be) to be blank as well. I know that if I put the formula =IF(B7="", "") in C7 and D7 it would return the correct results, but I want to have the ability to also type in C7 and D7 (so that people could enter their times in here if they are working that day. Can I do this with conditional formatting? If so, what would the formula be? I tried just entering the above formula into a conditional format rule, but it doesn't work.

Help!

Thanks,
Amanda

You're a genius! Thanks!