Moving Data to Empty Cell

I'm working in Excel 2002 and I have a spreadsheet with dates in column A, rates in column B, column C is empty, dates in D, rates in E, F empty, etc. I need to write a macro to move move the data in column D, E, G, H, etc to A and B so that all of the dates are in A and all of the rates are in B. The problem is that there are not always the same number of rows of data. How can I tell Excel to take all of the data from column D (however many rows that might be) and transfer it to column A starting with the first empty row (wherever that might be)? Hope this is clear. Thanks for the help!