How long ago was this?
Bash + GNU utilities, 37



tr , \\n|date -f- +%s|dc -e??-86400/p


tr replaces the comma with a newline.  date reads the newline separated dates and outputs the number of seconds since the Unix epoch that the passed-in date represents.  These numbers are then put on the dc stack.  Then its a simple matter of subtraction and divide by (24*60*60).  In this case, dc stack-based RPN arithmetic evaluation is better than bc or bash $( ), mostly because the subraction-before-division needs no parentheses.

Input via STDIN:

$ echo 2015-12-3,2015-12-1 | ./longago.sh
2
$ echo 2015-12-3,2014-12-1 | ./longago.sh
367
$ echo 2015-12-3,2013-12-3 | ./longago.sh
730
$