datapress doesn't like my spreadsheet

I am trying to load a Google spreadsheet into Datapress but keep getting rejected. The spreadsheet is here: https://docs.google.com/spreadsheet/ccc?key=0AgV8Hj2sidr8dEhiR0lyclN4cW53QkdBcGtJaHpuM1E
One possibility is the computed values in StartDate and EndDate.
I have used this spreadsheet with Exhibit with some success – http://uumarblehead.org/tricent/?page_id=943 – but want to use the plugin instead, since it’s a much cleaner approach.

Thanks in advance for whataever light you can shed on this

Hugh