Moment.js does not provide a way to set the time of an existing moment through a string. Why not just concatenate the two:

var date = "2017-03-13";
var time = "18:00";

var timeAndDate = moment(date + ' ' + time);

console.log(timeAndDate);

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Alternatively, you can use two Moment objects and use the getters and setters. Although a far more verbose option, it could be useful if you can't use concatenation:

let dateStr = '2017-03-13',
    timeStr = '18:00',
    date    = moment(dateStr),
    time    = moment(timeStr, 'HH:mm');

date.set({
    hour:   time.get('hour'),
    minute: time.get('minute'),
    second: time.get('second')
});

console.log(date);

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Since this pops up in google results, moment now has a set(unit, value) function and you can achieve this by:

const hours = 15;
const minutes = 32;
var date = moment("1946-05-21").set("hour", hours).set("minute", minutes);

or as a combined function

var date = moment("1946-05-21").set({"hour": 15, "minute": 32});

Note: the set function requires the value to be Integer type

Just incase anyone is wondering how to set the time on a Date Object, here's how I did it:

const dateObj = new Date();

const dateStr = dateObj.toISOString().split('T').shift();
const timeStr = '03:45';

const timeAndDate = moment(dateStr + ' ' + timeStr).toDate();