Hi James,
So you want a drop down for each; day, month, year? This is pretty easy to create manually. I would assume you would want to create the actual date selected at the end and you would just need to use concatenation to do this with rules.
The day and year are pretty easy but you would have to use a XML or list to convert the month's to the numeric values.
If you want a date picker to change then you would set an in an intermediate date like the first and then they could select the exact date via the picker.
Hope that helps