Age Calculator using HTML, Tailwind and JS Form Validation

I struggled with getting the calculated age to be consistent across different lengths of time. The approach I took was to:

1. Fetch the current time as a millisecond time stamp
2. Convert the user inputted date to a millisecond time stamp
3. Find the difference between the two time stamps
4. Convert that difference into a number of years by dividing by the average number of milliseconds in a year
5. Convert the remainder after calculating years into months by dividing by the average number of milliseconds in a month
6. Convert the remainder after calculating months into days by dividing by the number of milliseconds in a day
7. Use Math.floor() to round years, months and days down

The code for the calculation is as follows:

 function timestampToYMD(timestampDiff) {
        // calculated age is not accurate, try converting from ms to days first, then to months and years?
        const MsInADay = (1000 * 60 * 60 * 24)
        const NumOfLeapYears = (1000/4) - 7;
        const ExactMonthLength = 
        // Average length of a month, accounting for leap years using the number of leap years between 1000 and 2000
        (((1000 - ((1000/4) - 7)) * ((31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31) / 12) //average month length in a normal year
        + 
        (((1000/4) - 7) * (31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30 + 31) / 12) ) //average month length in a leap year
         / 1000 /* averaging out the normal year month length and leap year month length over the span of 1000 years */);
        const MsInAYear = (
            // taking years of 1001 to 2000
            /* LEAP YEAR RULES: 
                Divisible by 4
                For centuries, those divisible by 400
            */
            (365 + (NumOfLeapYears/ 999)) * MsInADay
        );
        // 31_557_016_216.21622
        let numOfYears = timestampDiff / MsInAYear;
        let numOfMonths = ((timestampDiff % MsInAYear) / (MsInADay * ExactMonthLength));
        let numOfDays = ((timestampDiff % MsInAYear) % (ExactMonthLength * MsInADay) / MsInADay);

        
        document.querySelector("#num-of-years").innerHTML = Math.floor(numOfYears);
        document.querySelector("#num-of-months").innerHTML = Math.floor(numOfMonths);
        document.querySelector("#num-of-days").innerHTML = Math.floor(numOfDays);
    }

This approach has a margin of error of a few days, so I'm wondering if there is a better approach to make the calculation more accurate. Would it be better to actually find the distance in days between two calendar dates than to calculate it using milliseconds?