Copy("0" + this.getDate()).slice(-2)

for the date, and similar:

Copy("0" + (this.getMonth() + 1)).slice(-2)

for the month.

I assume it's because it would be easier to reference in an array of names, i.e.

var months = ["January", "February", "March", "April", "May", "June", "July",
         "August", "September", "October", "November", "December"];

var d = new Date();

var namedMonth = months[d.getMonth()];

If getMonth() returned 1-12, then programmers would have to do d.getMonth()-1 everytime they wanted a fancy named month.

Days of the month don't have specific "names" per se. The getDate() returns 1-(28-31). We usually just refer to them by their number.

The same concept as getMonth() applies for getDay() also, which returns 0-6 based on the day of the week

var days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"];

var namedDay = days[d.getDay()];

All this returns something like:

console.log("Month: month[" + d.getMonth() + "]: " + namedMonth); 
//Month: month[3]:  April
console.log("Day: days[" + d.getDay() + "]: " + namedDay); 
// Day: days[4] : Thursday 

It is now possible to do this with the ECMAScript Internationalization API:

Copyconst date = new Date(2009, 10, 10);  // 2009-11-10
const month = date.toLocaleString('default', { month: 'long' });
console.log(month);

'long' uses the full name of the month, 'short' for the short name, and 'narrow' for a more minimal version, such as the first letter in alphabetical languages.

You can change the locale from browser's 'default' to any that you please (e.g. 'en-us'), and it will use the right name for that language/country.

With toLocaleString^api you have to pass in the locale and options each time. If you are going do use the same locale info and formatting options on multiple different dates, you can use Intl.DateTimeFormat instead:

Copyconst formatter = new Intl.DateTimeFormat('fr', { month: 'short' });
const month1 = formatter.format(new Date());
const month2 = formatter.format(new Date(2003, 5, 12));
console.log(`${month1} and ${month2}`); // current month in French and "juin".

For more information see my blog post on the Internationalization API.

Because getmonth() start from 0. You may want to have d1.getMonth() + 1 to achieve what you want.