Вы можете использовать эту функцию.
<script>
function getDateDifference(startDate, endDate) {
if (startDate > endDate) {
console.error('Start date must be before end date');
return null;
}
var startYear = startDate.getFullYear();
var startMonth = startDate.getMonth();
var startDay = startDate.getDate();
var endYear = endDate.getFullYear();
var endMonth = endDate.getMonth();
var endDay = endDate.getDate();
var february = (endYear % 4 == 0 && endYear % 100 != 0) || endYear % 400 == 0 ? 29 : 28;
var daysOfMonth = [31, february, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
var startDateNotPassedInEndYear = (endMonth < startMonth) || endMonth == startMonth && endDay < startDay;
var years = endYear - startYear - (startDateNotPassedInEndYear ? 1 : 0);
var months = (12 + endMonth - startMonth - (endDay < startDay ? 1 : 0)) % 12;
var days = startDay <= endDay ? endDay - startDay : daysOfMonth[(12 + endMonth - 1) % 12] - startDay + endDay;
return {
years: years,
months: months,
days: days
};
}
var result = getDateDifference(new Date("2018-01-01"), new Date("2019-01-02"));
//Print in console array with value
console.log(result);
var text = result.years+" year, "+result.months+" month, "+result.days+" days";
alert(text);
</script>
использовать getDateDifference
и параметры первая дата и вторая дата.В оповещении я вставляю демонстрационную версию.