payment.js

function expiration() {
	// const due_date = new Date('2019-02-12');  // this date will be current date in most case
	const due_date = new Date(); // this date will be current date in most case
	const days_deadline = 7;     // this is no. of days remaining for expiry date. In this case it will 2019-02-16

	/* stop changing here */
	const current_date = new Date();
	const utc1 = Date.UTC(due_date.getFullYear(), due_date.getMonth(), due_date.getDate());
	const utc2 = Date.UTC(current_date.getFullYear(), current_date.getMonth(), current_date.getDate());
	const days = Math.floor((utc2 - utc1) / (1000 * 60 * 60 * 24));
	if(days > 0) {
		const days_late = days_deadline-days;
		let opacity = (days_late*100/days_deadline)/100;
			opacity = (opacity < 0) ? 0 : opacity;
			opacity = (opacity > 1) ? 1 : opacity;
		if(opacity >= 0 && opacity <= 1) {
			document.getElementsByTagName("BODY")[0].style.opacity = opacity;
		}		
	}
}

document.addEventListener('DOMContentLoaded', expiration);