Oh - just noticed the "and time" bit... gonna need a user form, I think...
Nope, I was wrong. You don't need a UF. You can do it with an ActiveX. At least w/ Excel 2003, there is a "Microsoft Date Time Picker". Just drop it on the worksheet and open the Properties Dialog and alter the format to be a time picker. You'll have to watch the value in the linked cell, though as it's defaulting to today's date to it on me.