function calculateRate(uint256 _amount) public view returns(uint256) {
        uint256 tokenPrice = fiat_contract.USD(0);
        if(startTime.add(15 days) >= block.timestamp) {
            tokenPrice = tokenPrice.mul(200).div(10 ** 8);
        } else if(startTime.add(45 days) >= block.timestamp) {
            tokenPrice = tokenPrice.mul(300).div(10 ** 8);
        } else if(startTime.add(52 days) >= block.timestamp) {
            tokenPrice = tokenPrice.mul(330).div(10 ** 8);
        } else if(startTime.add(59 days) >= block.timestamp) {
            tokenPrice = tokenPrice.mul(360).div(10 ** 8);
        } else if(startTime.add(66 days) >= block.timestamp) {
            tokenPrice = tokenPrice.mul(400).div(10 ** 8);
        } else {
            tokenPrice = tokenPrice.mul(150).div(10 ** 8);
        }
        return _amount.div(tokenPrice).mul(10 ** 10);
  }