title: Match with substring in mongodb aggregation
body: <p>In my mongodb collection, I have a time_stamp=<b>"2013-06-30 23:58:37 928"</b>.
  I need to use "<b>$match</b>" with only the date, like time_stamp="<b>2013-06-30</b>". So how can I get the substring like that ?</p>
  
  <p>Previously I've tried with $substr, but it shows an error <b>"errmsg" : "exception: invalid operator: $substr"</b></p>