docs: function createDateTimeParser( formatKey, fallbackFormatsKey, isoFormat ) {
     return ( options, str ) => {
        const val = `${str}`.trim();
  
        if( val.length === 0 ) {
           return success( null );
        }
  
        let formats = [ options[ formatKey ] ];
        if( options[ fallbackFormatsKey ] && options[ fallbackFormatsKey ].length ) {
           formats = options[ fallbackFormatsKey ].concat( formats );
        }
  
        const mDate = momentParse( str, formats, options );
        return mDate.isValid() ? success( mDate.format( isoFormat ) ) : error();
     };
  }