Add 1 Day is timezone sensitive in moment

moment-timezone-test.js

function addDayTest(day, tz) {
  const start = moment.tz(day, 'YYYY-MM-DD', tz); 
  const plusDay = start.clone().add(1, 'day');      // Next calendar day at same time.
  const plusHours = start.clone().add(24, 'hours'); // 24 hours in the future.
  return {
     start: start.valueOf() / 1000,
     plusDay: plusDay.valueOf() / 1000,
     plusHours: plusHours.valueOf() / 1000,
     diff: (plusHours - plusDay) / 1000,   // How different are "1 day" and "24 hours"?
  };
}

addDayTest('2020-03-07', 'America/Chicago')
// Object { start: 1583560800, plusDay: 1583647200, plusHours: 1583647200, diff: 0 }

// Daylight savings time "spring forward" date in USA
addDayTest('2020-03-08', 'America/Chicago')
// Object { start: 1583647200, plusDay: 1583730000, plusHours: 1583733600, diff: 3600 }

// UTC observes no DST
addDayTest('2020-03-08', 'UTC')
// Object { start: 1583625600, plusDay: 1583712000, plusHours: 1583712000, diff: 0 }

// Phoenix (and most of Arizona) observe no DST
addDayTest('2020-03-08', 'America/Phoenix')
// Object { start: 1583650800, plusDay: 1583737200, plusHours: 1583737200, diff: 0 }