const date1 = new Date('7/13/2010');
const date2 = new Date('12/15/2010');
console.log(getDifferenceInDays(date1, date2));
console.log(getDifferenceInHours(date1, date2));
console.log(getDifferenceInMinutes(date1, date2));
console.log(getDifferenceInSeconds(date1, date2));

function getDifferenceInDays(date1, date2) {
  const diffInMs = Math.abs(date2 - date1);
  return diffInMs / (1000 * 60 * 60 * 24);
}

function getDifferenceInHours(date1, date2) {
  const diffInMs = Math.abs(date2 - date1);
  return diffInMs / (1000 * 60 * 60);
}

function getDifferenceInMinutes(date1, date2) {
  const diffInMs = Math.abs(date2 - date1);
  return diffInMs / (1000 * 60);
}

function getDifferenceInSeconds(date1, date2) {
  const diffInMs = Math.abs(date2 - date1);
  return diffInMs / 1000;
}

let today = new Date().toISOString().slice(0, 10)

const startDate  = '2021-04-15';
const endDate    = today;

const diffInMs   = new Date(endDate) - new Date(startDate)
const diffInDays = diffInMs / (1000 * 60 * 60 * 24);


alert( diffInDays  );

const date1 = new Date('19/1/2021');
const date2 = new Date('20/1/2021');
const diffTime = Math.abs(date2 - date1);
const diffDays = Math.ceil(diffTime / (1000 * 60 * 60 * 24)); 
console.log(diffTime + " milliseconds");
console.log(diffDays + " days");

const diffDays = (date, otherDate) => Math.ceil(Math.abs(date - otherDate) / (1000 * 60 * 60 * 24));

// Example
diffDays(new Date('2014-12-19'), new Date('2020-01-01'));   // 1839

function javascript (date1, date2) {
   let oned = 24 * 60 * 60 * 1000;
   return Math.ceil((date2 - date1) / oned);
}

function javascript(date1, date2) {
	return new Date(date2 - date1).getDate() - 1
}

function javascript(date1, date2) {
  return Math.ceil((date2 - date1) / 8.64e7);
}

/* difference between date1 and date2 in days (date2 - date1) */
/* date1 and date 2 are already javascript date objects */
function dateDifference(date2, date1) {
    const _MS_PER_DAY = 1000 * 60 * 60 * 24;

    // Discard the time and time-zone information.
    const utc1 = Date.UTC(date1.getFullYear(), date1.getMonth(), date1.getDate());
    const utc2 = Date.UTC(date2.getFullYear(), date2.getMonth(), date2.getDate());

    return Math.floor((utc2 - utc1) / _MS_PER_DAY);
}