function rate() public constant returns (uint256) {
    if (block.timestamp < startTime) return 0;
    else if (block.timestamp >= startTime && block.timestamp < (startTime + 1 weeks)) return uint256(default_rate/2);
    else if (block.timestamp >= (startTime+1 weeks) && block.timestamp < (startTime + 2 weeks)) return uint256(10*default_rate/19);
    else if (block.timestamp >= (startTime+2 weeks) && block.timestamp < (startTime + 3 weeks)) return uint256(10*default_rate/18);
    return 0;
  }