function getYearDistance(start, end) {
       return end.getFullYear() - start.getFullYear();
     }
Get the integer distance between two years. This *only* considers the 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.