It is giving you midnight local time, but you’re printing it out in zulu time. Try using toString instead, it will print the time out in local time.
now = moment()
console.log('now ' + now.toString())
console.log('start ' + now.startOf('day').toString())
console.log('end ' + now.endOf('day').toString())<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.22.1/moment.js"></script>