Using xtype datefield not to select past date in CQ5

You need to write custom js and listeners to implement this.

(1) Create a js function to compare dates:\

/**
*Compares dates and return values in 0,-1,1,2
**/
MyUtility.compareDate = function(a_dateOne ,a_dateTwo)
{                       var returnVal = 'default';
                        var l_dateOne;
                        var l_dateTwo;
                            l_dateOne = a_dateOne.split('/');
                            l_dateTwo = a_dateTwo.split('/');                      
                        if(l_dateOne[2] == l_dateTwo[2] && l_dateOne[1] == l_dateTwo[1] && l_dateOne[0] == l_dateTwo[0]){
                            returnVal = 0;
                        }
                        else if(l_dateOne[2] > l_dateTwo[2]){
                            returnVal = 1 ;
                        }else if(l_dateOne[2] == l_dateTwo[2]){
                            if(l_dateOne[1] > l_dateTwo[1]){
                                returnVal = 1;
                            }else if(l_dateOne[1] == l_dateTwo[1]){
                                if(l_dateOne[0] > l_dateTwo[0]){
                                    returnVal = 1;
                                }else{
                                    returnVal = -1;
                                }
                            }else{
                                returnVal = -1;
                            }
                        }else{
                            returnVal = -1;
                        }
                        return returnVal;
}

(2) Call this function using CQ listeners beforeSubmit event.

function(){
var currentDate = new Date();
currentDate=currentDate.format("d/m/y");
var eventDate=this.findByType('datefield')[0].getDateValue();
var event_year = new Date(eventDate).getFullYear();
var current_year = new Date(currentDate).getFullYear();

if(currentDate!=null)
{
    eventDate=eventDate.format("d/m");
    currentDate=currentDate.format("d/m");
    eventDate = eventDate.concat("/"+event_year);
    currentDate = currentDate.concat("/"+current_year);
    var l_expirationDateCheck=MyUtility.compareDate(eventDate,currentDate);
    if(l_expirationDateCheck==1)
                    {
                     var msg = CQ.I18n.getMessage("Selected Date cannot be less than current date");
                        CQ.Ext.Msg.show({
                            "title": CQ.I18n.getMessage("Error Message"),
                            "msg": msg,
                            "buttons": CQ.Ext.Msg.OK
                        });
                        return false;
                    }
}
}