Creating a class that contains only static properties and methods is a good way to modularize utility functions, placing them contextually into thematic groups. This prevents the global namespace from being polluted. For example:

export default class DateUtils {
 static addDays(date, numDays) {
    return new Date(date.valueOf() + (numDays*24*60*60*1000));
 }
 static diffDays(date) {
    if (date.constructor.name == 'String')
     date = new Date(date);
    var nowTS = new Date().valueOf();
    var dateTS = date.valueOf();
    return Math.ceil((dateTS - nowTS) / (24*60*60*1000));
 }
 static format(date, fmt) {
   if (date == null)
      return '';
   if (date.constructor.name == 'String')
      date = new Date(date);
   if (Number.isNaN(date.getFullYear()) ||
       Number.isNaN(date.getMonth()) ||
       Number.isNaN(date.getDate()))
         return '';
   switch (fmt) {
     case 'YYYY-MM-DD':
       let YYYY = (date.getFullYear()).toString();
       let MM   = (date.getMonth() + 1).toString().padStart(2, '0');
       let DD   = (date.getDate()).toString().padStart(2, '0');
       return `${YYYY}-${MM}-${DD}`
    case 'Month Day, Year':
       let YYYY = (date.getFullYear()).toString();
       let MM   = ['Jan','Feb','Mar','Apr','May','Jun','Jul','Aug',
                   'Sep','Oct','Nov','Dec'][date.getMonth()];
       let DD   = (date.getDate()).toString();
       return `${MM} ${DD}, ${YYYY}`
    default:
       throw new Error(`unhandled date format ${fmt}`);
    }
  }
}