Cannot Group in Pivot Table

I am trying to convert this data, sourced from a text file that was opened in Excel 2010 on Windows 7. The data is simply dates and rainfall totals for each day. The data is primarily daily rainfall data but there are some monthly totals in there too. I want to end up with monthly TOTALS for each month of each year. When I get to the step of grouping the pivot table dates into months, it won't allow me to group. I have formatted the dates as dates, and have even replaced the / in the dates in case they were mess sign things up. there are no blank cells in the data. I think that maybe Excel is not recognizing the dates as dates even though they are formatted that way...can anyone help me solve this problem? Here is the file..
thank you