Aggregating by date field
I recommend trying the Views Aggregator Plus module. It supports many useful aggregation functions. As an exmaple creating the following output:


Industry    Companies             Turnover
Food (2)    Heiny, McRonalds      $566,000,000
Clothing (1)Cenneton              $99,000,000
IT (2)      AcquiB, PreviousBest  $30,000,000
Totals              5             $695,000,000


From this source table:


Industry    Company Name    Turnover
IT          AquiB           $25,000,000
Clothing    Cenneton        $99,000,000
Food        Heiny           $66,000,000
IT          PreviousBest    $5,000,000
Food        McRonalds       $500,000,000



  With the grouping taking place on, say Industry, and aggregation
  functions TALLY, ENUMERATE and SUM applied on Industry, Company Name
  and Turnover respectively, the final result with a descending sort on
  the last column will display like below. Note that the module
  identified three groups. It also added a totals row, Views Calc style.