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

I played around with some made-up dates and numbers and got the Group option to work, so the issue must be with the formatting of the data somehow. Also, is there a way to sort the data into monthly data that also shows the year AND month for each year?

I have also done all these things (from another website)...not sure how to find spaces in my thousands of rows of dates though. I have no blank cells.


Problems when Grouping Data[/B]

If you try to group a date or number field, you may see an error message that says, "Cannot group that selection."

This problem usually occurs when the field contains records with a blank date/number field, or text in a date/number field. To fix the problem

For blank cells, fill in the date/number (use a dummy date/number if necessary).
If there is text in the date/number field, remove it.
If numbers are being recognized as text, use one of the techniques to change text to real numbers. Then return to this pivot table tutorial, and try the grouping steps.

If you don't have blank cells or text in the date column, there may be a grouped field left over from the previous time that you grouped the data.

Check the field list, to see if there's a second copy of the date field, e.g. Date2.
If there is, add it to the row area, and ungroup it.
Then, you should be able to group the date field again.