Views math expression or Computed field to find date difference in views
You can achieve this use a template file, since you can use PHP inside templates in Drupal 7

In your view



Now, add both of your date fields and do this to each of them



In your web directory, go to /sites/all/themes/[yourtheme]/templates folder

Create a file named views-view-fields--[view-name]--[machine-name].tpl.php

EX: views-view-fields--hello-world--block-1.tpl.php

Inside the template file put:

&lt;?php

$date1 = new DateTime('$fields["field_invoice_issue_date"]-&gt;content');
$date2 = new DateTime('$fields["field_payment_receipt_date"]-&gt;content');
$interval = $date1-&gt;diff($date2);
echo "difference " . $interval-&gt;y . " years, " . $interval-&gt;m." months, ".$interval-&gt;d." days "; 

// shows the total amount of days (not divided into years, months and days like above)
echo "difference " . $interval-&gt;days . " days ";

?&gt;


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