function _getTokenBonus(uint256 _weiAmount) internal view returns (uint256) {
        uint256 bonusRate = 0;
        if(block.timestamp > openingTimePeriodOne && block.timestamp < closingTimePeriodOne){
            bonusRate = bonusRatePeriodOne;
        } else if(block.timestamp > openingTimePeriodTwo && block.timestamp < closingTimePeriodTwo){
            bonusRate = bonusRatePeriodTwo;
        }
        return _weiAmount.mul(10 ** uint256(decimals)).div(1 ether).mul(bonusRate);
    }