Calendar getInstance() shows wrong date randomly

Revision 1
In my TPlayer class

private String getDate() {
		DateFormat dateFormat = new SimpleDateFormat("MM/dd/yyyy HH:mm");

        return dateFormat.format(lastLoginCal.getTime());
    }

In the event listener

@EventHandler(priority = EventPriority.NORMAL)
	public void onPlayerJoin(PlayerJoinEvent e)
	{
		Player player = e.getPlayer();
                TPlayer tplayer = TEssentials.getTPlayers().getPlayer(player.getName());
                tplayer.setLastLoginCal(Calendar.getInstance());
	}

I save the last login string to a MySQL database.
The problem is. 9 out of 10 show the correct date but one of the 10 are showing next month and one out of 50 are showing 2 months ahead.

exp: Todays date 1/18/2013 in MM/dd/yyy
player bob 01/18/2013
Player joe 01/18/2013
Player susi 01/18/2013
...
Player sam 02/18/2013
Player jill 01/18/2013
...
Player matt 03/18/2013

Im not seeing why my code would return a invalid date. Any help would be nice :D
And the MySQL save just does a UPDATE and gets each TPlayer from a Map