JavaScript has own dedicated Date object.  However it does not provide own formatting options so there are plenty of examples available that show different approaches on printing a formatted date.  Here we show one of them covering formatting as ISO8601 string. You may modify this function to fit your own needs:

// Put this function into Function (.user.js) file 
function toISOString(date)
{
    date = date || new Date();
    function pad(number)
    {
        var r = String(number);
        if (r.length === 1)
        {
            r = '0' + r;
        }
        return r;
    }
    return date.getFullYear()
        + '-' + pad(date.getMonth() + 1)
        + '-' + pad(date.getDate())
        + 'T' + pad(date.getHours())
        + ':' + pad(date.getMinutes())
        + ':' + pad(date.getSeconds())
        + '.' + String((date.getMilliseconds() / 1000).toFixed(3)).slice(2, 5)
        + 'Z';
}

 

1. Print Today's date in ISO8601 format:

// Now you may use it like that:
// Current date:
var today = new Date();
var isoDate = toISOString(today)
Tester.Message("Iso date: " + isoDate);

2. Print Today's date in short format yyyy-mm-dd:

// Short version of the date - cut 1st 10 symbols so we have yyyy-mm-dd:
var today = new Date();
var yyyymmdd= toISOString(today).substring(0, 10);
Tester.Message("yyyymmdd: " + yyyymmdd);

3.  Print specific date:

// Specific date (2015-04-01). 
// Note that month is 0-based, so 0 is Jan, 3 is April.
var someDate = new Date(2015, 3, 1);
Tester.Message("Specific date: " + toISOString(someDate));

4.  Print specific date and time:

// Specific date and time (2012-10-05T20:45). 
// Note that month is 0-based, so 0 is Jan, 3 is April.
var someDateTime = new Date(2012, 9, 5, 20, 45);
Tester.Message("Specific date and time: " + toISOString(someDateTime));