Updating a date automatically from another date

[Moedeb]

Ok so I have two variable date fields:

1. start_date
2. end_date

Once the start date is entered I want the end date to automatically be updated to 90 days after the start date.

So I have added an on change catalog client script as follows:

Type - onChange

Variable Name - start_date

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    //Type appropriate comment here, and begin script below
    var addDays = "90";

    var StartDate = new Date(g_form.getValue('sta_start_date_approved'));

    var addDaysToStartDate = "";
    var getDateFromStartDate = StartDate.getDate();

    var totalDays = parseInt(getDateFromStartDate) + parseInt(addDays);
    StartDate.setDate(totalDays);

    addDaysToStartDate = formatDate(StartDate, g_user_date_format);

    g_form.setValue('sta_end_date', g_user_date_format);
}

This is sort of working, but what is happening is it is not using the correct date format.

What I noticed when I changed the var addDays = "90" to only = 3, then I could see what it was doing was ultimately changing the date format of the end_date field.

eg:

my start_date uses the format dd/MM/yyyyy as should the end_date, But

when it added the 3 days it would move the dd to be where the MM is and add 3 days to the MM instead.

This sounds very complicated sorry.

So this date: 01/12/2023 (meaning 1 Dec 2023) would change to be 12/04/2023 - so the 3 days when using the date format of dd/MM/yyyy was really updating the month thinking it was the days.

I hope that makes sense to someone who can help me? 

I need the date format to stay the correct way in the end_date field and add the days to the correct part of the date.

[Moedeb]

Solution provided by @-O- 

function onChange (control, oldValue, newValue, isLoading) {
if (isLoading || newValue == '') {
return;
}
var addDays = 90;
// Calcualte how many milliseconds the desired number of days contain
var offsetMilliseconds = addDays * 24 * 60 * 60 * 1000;

// Get the value in field (make sure to cut off the time part - if the field is in fact a date/time vs. date)
var startDateString = g_form.getValue('sta_start_date_approved').substr(0, g_user_date_format.length);

// Have ServiceNow API compute Unix epoch milliseconds out of the date string
var startDateValue = getDateFromFormat(startDateString, g_user_date_format);

// Calculate hos mony milliseconds the end date contains
var endDateValue = startDateValue + offsetMilliseconds;

// Turn the Unix epoch milliseconds into a Date
var endDate = new Date(endDateValue);

// Set the end date value
g_form.setValue('sta_end_date', formatDate(endDate, g_user_date_format));
}

[Moedeb]

So for whatever reason the solution provided by @-O-  won't save as text / code here on the community, but it certainly works.

Here is a screenshot of the code for others in the future potentially

[Moedeb]

Here is the solution provided by @Tai Vu 

function onChange(control, oldValue, newValue, isLoading) {
    if (isLoading || newValue == '') {
        return;
    }

    //Type appropriate comment here, and begin script below
    var addDays = 90;
	var StartDate = getDateFromFormat(g_form.getValue('sta_start_date_approved'), g_user_date_format);

	StartDate = new Date(StartDate); //Add this line to your script

    var addDaysToStartDate = "";
    var getDateFromStartDate = StartDate.getDate();

    var totalDays = parseInt(getDateFromStartDate) + addDays;
    StartDate.setDate(totalDays);

    addDaysToStartDate = formatDate(StartDate, g_user_date_format);

    g_form.setValue('sta_end_date', addDaysToStartDate);

}

[Moedeb]

This has become easier to do now using a flow with no coding required.

Here is a link to the instructions:

How to add days to a date field in a flow with no coding