title: Trouble converting SUMIFS to SUMPRODUCT
body: <p>I am trying to rewrite a <code>SUMIFS</code> formula as <code>SUMPRODUCT</code> so that it can reference closed worksheets. The formula checks a column with dates if the date is between two values, and if so sums another column. 
  The <code>SUMIFS</code> formula is</p>
  
  <pre><code>  =SUMIFS('WorkbookName'!$D:$D,'WorkbookName'!$A:$A
    "&gt;=7/1/2015",'WorkbookName'!$A:$A"&lt;=9/30/2015")
  </code></pre>
  
  <p>This works as long as both workbooks are open, however I cannot get my <code>SUMPRODUCT</code> to produce anything other than a <code>"-"</code>
  THis is what I have tried (where <code>A17</code> and <code>A18</code> are the two dates above)</p>
  
  <pre><code>=SUMPRODUCT(--('Workbook'!$A:$A&gt;=A17),('Workbook'!$A:$A&lt;=A18) ('Workbook'!$C:$C))
  </code></pre>