function ISODateString(d){
  function pad(n){
    return n < 10 ? '0' + n : n;
  }
  return d.getUTCFullYear()+'-'
    + pad(d.getUTCMonth()+1)+'-'
    + pad(d.getUTCDate())+'T'
    + pad(d.getUTCHours())+':'
    + pad(d.getUTCMinutes())+':'
    + pad(d.getUTCSeconds())+'Z';
}
Accepts a `Date` instance and returns an ISO date string.

@param {Date} d - Date instance to serialize
@returns {String} ISO date string representation of `d`
@api private