Difference of points in time

Let's find the difference between the two points in time:

let date1 = new Date();
let date2 = new Date(2015, 4, 25, 12, 59, 59);

console.log(date1 - date2); // difference in milliseconds

Determine the number of days between January 1st and September 10th of the current year.

Determine the number of days between 20th of the current month and 10th of the next month.