Grouping dates - Formatting Issue?

Hi,

I'm trying to create a pivot table on the data in the attached file and grouping the expenses into months by right clicking on the data column and selecting "Group". EXCEL gives me an error message "Cannot group that selection". I'm pretty sure it's something to do with the format of the date column (if I take a sample and manually change the dates to standard xx/xx/xxxx format it works fine). I've included the basic download (from a mobile phone) and the pivot table. If anyone can explain to me what the issue is and the solution I would be very grateful,

regards, Neil