convertElapsedTimeToTimeIndex
  = function convertElapsedTimeToTimeIndex(
    currentTime,
    startTime,
    aggregateTimeUnits
  ) {
    return Math.floor((currentTime - startTime) / aggregateTimeUnits);
  }
Given a moment in time, the start time, and time units, produce the
correct time index.

@param {Number} currentTime
The current time.

@param {Number} startTime
The start time, to derived elapsed time.

@param {Number} aggregateTimeUnits
The time units to derive the index.

@returns {Number}
The time index for the elapsed time is returned.