Revert "Fix performance issues with wantsDateSeperator"

This change broke timezone handling, so that date-separators are shown at the
wrong time of day.

This reverts commit b908e7e.

Author: richvdh

src/components/structures/MessagePanel.js

@@ -21,8 +21,6 @@ var sdk = require('../../index');
 
 var MatrixClientPeg = require('../../MatrixClientPeg')
 
-const MILLIS_IN_DAY = 86400000;
-
 /* (almost) stateless UI component which builds the event tiles in the room timeline.
  */
 module.exports = React.createClass({
@@ -477,7 +475,9 @@ module.exports = React.createClass({
             // here.
             return !this.props.suppressFirstDateSeparator;
         }
-        return Math.floor(prevEvent.getTs() / MILLIS_IN_DAY) !== Math.floor(nextEventTs / MILLIS_IN_DAY);
+
+        return (new Date(prevEvent.getTs()).toDateString()
+                !== new Date(nextEventTs).toDateString());
     },
 
     // get a list of read receipts that should be shown next to this event