- else if (elapsed > duration)
+ else if (elapsed >= duration)
proportionNow =
  uint256(initialProportion) +
  uint256(1e18 - initialProportion).wmul(elapsed.wdiv(duration));

// = initialProportion + (1e18 - initialProportion).wmul(1e18)
// = initialProportion + (1e18 - initialProportion) * 1e18 / 1e18
// = initialProportion + 1e18 - initialProportion
// = 1e18
