console.log(start);
const parsed = start.format('YYYY-MM-DD HH:mm:ss')
console.log(parsed);

start: Moment
_ambigTime: true
_ambigZone: true
_d: Tue Dec 11 2018 20:00:00 GMT-0500 (Eastern Standard Time) {}
_fullCalendar: true
_i: Wed Dec 12 2018 07:00:00 GMT-0500 (Eastern Standard Time) {}
_isAMomentObject: true
_isUTC: true
_isValid: true
_locale: e {_weekdaysShort: Array(7), _fullCalendar_weekCalc: "local"}
_offset: 0
_pf: {empty: false, unusedTokens: Array(0), unusedInput: Array(0), overflow: -2, charsLeftOver: 0, …}

"2018-12-12 00:00:00"