function mintTokens(address beneficiary, uint256 tokens) internal {
        uint256 weiAmount;
        if (block.timestamp >= time0 && block.timestamp < time2) weiAmount = tokens.div(11000);
        else if (block.timestamp >= time3 && block.timestamp < time7) weiAmount = tokens.div(10000);

        weiRaised = weiRaised.add(weiAmount);
        token.mint(beneficiary, tokens);
        addNewHolder(beneficiary);
        TokenPurchase(msg.sender, beneficiary, weiAmount, tokens, block.timestamp);
    }