Create a custom calculation script for age in months that doesn't round up

The issue is that your current formula calculates the difference in years and months but doesn't check the day of the month. That's why it shows 23 months even when the child is only 22 months and 28 days old.

You need to subtract one month whenever the current day is earlier than the birth day.

Try replacing your calculation with this:

var cDateFormat = "yyyy-mm-dd";
var cDate = this.getField("dob").value;

if (cDate == "") {
    event.value = "";
} else {
    var oDate = util.scand(cDateFormat, cDate);

    if (oDate == null) {
        app.alert("Please enter a valid date of birth.");
        event.value = "";
    } else {
        var now = new Date();

        var totalMonths =
            (now.getFullYear() - oDate.getFullYear()) * 12 +
            (now.getMonth() - oDate.getMonth());

        // Subtract one month if the current day hasn't reached
        // the birth day yet.
        if (now.getDate() < oDate.getDate()) {
            totalMonths--;
        }

        event.value = Math.max(0, totalMonths);
    }
}

For example, if the birth date is January 15 and today's date is November 13 of the following year, the result will be 21 completed months, not 22.

This calculates completed calendar months rather than rounding up partial months.

One edge case to consider: if the birth date is January 31 and the current date is February 28, this strict calculation returns 0 completed months. You may want to decide whether the last day of February should count as one completed month in your form.

For testing your expected age calculations against different birth dates, you can also compare the results with this <a href="https://hisaabage.com/age-calculator/">online age calculator</a>, which displays age in years, months and days.

Hope this helps!

This isn't an issue of rounding at all. Your code has to do what YOU would do in giving the child's age.  For example. The child was born in January. It is 20 July. How old are they? Can YOU answer that question?  Or do you need more info? And, if you need more info, what EXACTLY do you do with it?  The code has to match what a human would do.

Look also at the day of month.

in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post if it helps you get responses.

<moved from using the community>