Converting a UTC datetime to EDT timezone using date-fns.js (without using an IANA db)

EDT_date.js

import parse from 'date-fns/parse';
import format from 'date-fns/format';
import isValid from 'date-fns/is_valid';
import addMinutes from 'date-fns/add_minutes';

export default function (datetime) {
  const parsed = parse(datetime);

  if (!isValid(parsed)) { // return empty & log error for invalid dates
    console.error('invalid date:', datetime);
    return '';
  }

  const utcOffset = parsed.getTimezoneOffset();
  const isDST = (utcOffset < new Date(2016, 1, 1).getTimezoneOffset());
  const edtOffset = isDST ? 240 : 300; // 300mins or 240mins during Daylight savings
  const offsetDiff = utcOffset - edtOffset;
  let adjustedTime = parsed;
  if (offsetDiff) {
    adjustedTime = addMinutes(parsed, offsetDiff);
  }

  return `${format(adjustedTime, 'MMM.DD.YYYY / h:mm A [ET]')}`;
}