var date = '2018-09-12T00:00:00Z';
var time = '16:00:00';
var momentDate = moment(date);
var momentTime = moment(time, 'HH:mm:ss');
momentDate = momentDate.set({
hour: momentTime.get('hour'),
minute: momentTime.get('minute'),
second: momentTime.get('second')
});
console.log(momentDate.format());
console.log(momentDate.isAfter(moment.now()));