Just figure out the difference in seconds (don’t forget JS timestamps are actually measured in milliseconds) and decompose that value:
// get total seconds between the times
var delta = Math.abs(date_future - date_now) / 1000;
// calculate (and subtract) whole days
var days = Math.floor(delta / 86400);
delta -= days * 86400;
// calculate (and subtract) whole hours
var hours = Math.floor(delta / 3600) % 24;
delta -= hours * 3600;
// calculate (and subtract) whole minutes
var minutes = Math.floor(delta / 60) % 60;
delta -= minutes * 60;
// what's left is seconds
var seconds = delta % 60; // in theory the modulus is not required
EDIT code adjusted because I just realised that the original code returned the total number of hours, etc, not the number of hours left after counting whole days.