function calcAmountAt(uint256 _value, uint256 at) public constant returns (uint256) {
        uint rate;

        if(startTime + 2 days >= at) {
            rate = 140;
        } else if(startTime + 7 days >= at) {
            rate = 130;
        } else if(startTime + 14 days >= at) {
            rate = 120;
        } else if(startTime + 21 days >= at) {
            rate = 110;
        } else {
            rate = 105;
        }
        return ((_value * rate) / weiPerToken) / 100;
    }