PivotTables, Calculated Fields, Grouping, and #DIV/0! errors

Greetings all, I have a pivottable, that gives me count values for a set of data with various dates in it: I also have a calculated field that gives me a ratio formulated as follows: EER = 1 - RedoDate / ReleaseDate.

When the data is not grouped, everything works fine, and I get the calculated field EER to calculate. However, when I group by say, Monday, the calculated field = #DIV/0! for all instances. I can manually group by selecting the range for a given month, but I cannot "Group" by Month or Year, or Quarter, etc.

Any ideas are greatly appreciated? Thanks,

PivotTable1.jpgPivotTable2.jpg

Please find sample sheet attached.