function _getbonusRate()
      internal view returns (uint256)
    {
        
        if(_tier2Start > block.timestamp){
            return(_baseRate * 6 / 5);
        }
        else if(_tier3Start > block.timestamp){
            return(_baseRate * 11 / 10);
        }
        else if(_tier4Start > block.timestamp){
            return(_baseRate * 21 / 20);
        }
        else {
            return(_baseRate);
        }
    }