public Date getMinDate(){
	    	Date ret = null;
	    	for (Integer key: colMinDates.keySet()) {
	    		Date d = colMinDates.get(key);
	    		if (ret == null || ret.after(d)){
	    			ret = d;
	    		}	    		 
	    	}
	    	return ret;	    	
	    }