別に使いこなさなくても時刻0埋めはIntlAPIで簡単にできる

date = new Date('2018/06/01 01:02:03')

date.toLocaleTimeString( 'en', { hour12: false } )
// "01:02:03"

date.toLocaleTimeString( 'en', { minute: '2-digit', second: '2-digit', hour12: false } )
// "02:03"