function getMonthDistance(start, end) {
      return (12 * (end.getFullYear() - start.getFullYear())) + (end.getMonth() - start.getMonth());
    }
Get the integer distance between two months. This *only* considers the month and year
portion of the Date instances.

@param {Date} start
@param {Date} end
@returns {number} Number of months between `start` and `end`. If `end` is before `start`
    chronologically, this number will be negative.