In date.html add these new lines of JavaScript to create a new date, display the date part and change it before displaying it again:

var anotherDate=new Date('January 16, 2028');
alert(anotherDate.getDate());
anotherDate.setDate(13);
alert(anotherDate.getDate());

In this code you have created a new date object containing the date you chose rather than the current one.  Note you didn't bother saying what time it was so assumptions will be made.  On the second line you display the date part before changing that on the third line and displaying the new value.

You can also set the other parts of the date within the object using setDate(), setMonth() etc..

If you prefer to manipulate the date as a string you can use the toString() method.