how many days in between code example

Example 1: calculate days between dates

let startDate = "2021-04-01";
let date1 = new Date();
let date2 = new Date(startDate);
let timeInMilisec = date1.getTime() - date2.getTime();
let daysBetweenDates = Math.ceil(timeInMilisec / (1000 * 60 * 60 * 24));

Example 2: get days between two dates

// new Date("dateString") is browser-dependent and discouraged, so we'll write
// a simple parse function for U.S. date format (which does no error checking)
function parseDate(str) {
    var mdy = str.split('/');
    return new Date(mdy[2], mdy[0]-1, mdy[1]);
}

function datediff(first, second) {
    // Take the difference between the dates and divide by milliseconds per day.
    // Round to nearest whole number to deal with DST.
    return Math.round((second-first)/(1000*60*60*24));
}

alert(datediff(parseDate(first.value), parseDate(second.value)));