Create ISO 8601 format date strings in Javascript

constructISOString.js

/**
 * @param {number} unixTime - seconds elapsed since 1970-01-01T00:00:00.000Z
 * @returns {string} - ISO 8601 format date, not including milliseconds
 */
export const constructISOString = function (unixTime) {
  const date = new Date(unixTime * 1000);

  const fourDigitYear = date.getFullYear().toString().padStart(4, '0');
  const twoDigitMonth = (date.getMonth() + 1).toString().padStart(2, '0');
  const twoDigitDate = date.getDate().toString().padStart(2, '0');

  const twoDigitHours = date.getHours().toString().padStart(2, '0');
  const twoDigitMinutes = date.getMinutes().toString().padStart(2, '0');
  const twoDigitSeconds = date.getSeconds().toString().padStart(2, '0');

  const timezoneOffset = date.getTimezoneOffset();

  const timezoneOffsetAbsolute = Math.abs(timezoneOffset);
  const timezoneOffsetIsNegative = timezoneOffset === 0 || Math.sign(timezoneOffset) === -1;

  const twoDigitTimezoneOffsetHours = Math.trunc(timezoneOffsetAbsolute / 60).toString().padStart(2, '0');
  const twoDigitTimezoneOffsetMinutes = Math.trunc(timezoneOffsetAbsolute % 60).toString().padStart(2, '0');

  return `${fourDigitYear}-${twoDigitMonth}-${twoDigitDate}T${twoDigitHours}:${twoDigitMinutes}:${twoDigitSeconds}${timezoneOffsetIsNegative ? '+' : '-'}${twoDigitTimezoneOffsetHours}:${twoDigitTimezoneOffsetMinutes}`;
};