var event = new Date('August 19, 1975 23:15:30');
event.setHours(20);

console.log(event);
// expected output: Tue Aug 19 1975 20:15:30 GMT+0200 (CEST)
// (note: your timezone may vary)

event.setHours(20,21,22);

console.log(event);
// expected output: Tue Aug 19 1975 20:21:22 GMT+0200 (CEST)