ISO8601 date & time with offset for Javascript

dateToJson.js

Date.prototype.toJSON = function () {
  const tzOffset = 0 - this.getTimezoneOffset();  // Javascript is backwards from ISO8601
  const sign = tzOffset >= 0 ? '+' : '-';
  const timezoneOffsetInHours = Math.abs(Math.floor(tzOffset / 60));
  const leadingHourZero = timezoneOffsetInHours < 10 ? '0' : '';
  const timezoneOffsetMinuteRemainder = Math.abs(Math.floor(tzOffset % 60));
  const leadingMinuteZero = timezoneOffsetMinuteRemainder < 10 ? '0' : '';
  const offset =  sign + leadingHourZero + timezoneOffsetInHours + ':' + leadingMinuteZero + timezoneOffsetMinuteRemainder;

  const fakeDate = new Date(Date.UTC(this.getFullYear(), this.getMonth(),
    this.getDate(), this.getHours(), this.getMinutes(), this.getSeconds()));
  const iso = fakeDate.toISOString().substring(0,19);

  return iso + offset;
};

dateToJSON.spec.js

import "./dateToJSON";

// This spec file should be exercised in various timezones. Set the timezone in the container with
// docker exec -it -e TZ=Pacific/Chatham cmd

// Or interactively with commands like the following:
// echo "Pacific/Niue" > /etc/timezone
// dpkg-reconfigure tzdata
// date

describe('Date Prototype', () => {
  describe('toJSON', () => {
    const localNow = new Date();
    const localNowToJSON = localNow.toJSON();

    it('Date().toJSON() overrides the usual JSON.stringify() behavior (which is used by Axios for posting to API endpoints)', () => {
      const expectedResult = '"' + localNowToJSON + '"';
      const result = JSON.stringify(localNow);
      expect(result).toEqual(expectedResult);
    });
    it('new Date().toJSON() produces a string', () => {
      // act
      const result = new Date().toJSON();

      // assert
      expect(result).toEqual(expect.any(String));
    });
    it('produces an ISO8601 timestamp with offset', () => {
      // arrange
      const now = new Date();
      // 2000-01-24T02:34:56-10:00
      const iso8601pattern = /\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}[+-]\d{2}:\d{2}/;

      // act
      const result = now.toJSON();

      // assert
      expect(result).toMatch(iso8601pattern);
    });
    it('matches the local year', () => {
      // arrange
      const expectedResult = localNow.getFullYear();

      // act
      const result = parseInt(localNowToJSON.substring(0,4), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('matches the local month', () => {
      // arrange
      const expectedResult = localNow.getMonth() + 1;

      // act
      const result = parseInt(localNowToJSON.substring(5,7), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('matches the local date', () => {
      // arrange
      const expectedResult = localNow.getDate();

      // act
      const result = parseInt(localNowToJSON.substring(8,10), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('matches the local hour', () => {
      // arrange
      const expectedResult = localNow.getHours();

      // act
      const result = parseInt(localNowToJSON.substring(11,13), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('matches the local minutes', () => {
      // arrange
      const expectedResult = localNow.getMinutes();

      // act
      const result = parseInt(localNowToJSON.substring(14,16), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('matches the local seconds', () => {
      // arrange
      const expectedResult = localNow.getSeconds();

      // act
      const result = parseInt(localNowToJSON.substring(17,19), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('includes the correct local UTC offset sign', () => {
      // arrange
      const expectedResult = 0 - localNow.getTimezoneOffset() >= 0 ? '+' : '-';
      // act
      const result = localNowToJSON.substring(19,20);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('includes the local UTC offset hours', () => {
      // arrange
      const expectedResult = Math.floor(Math.abs(localNow.getTimezoneOffset() / 60));
      // act
      const result = parseInt(localNowToJSON.substring(20,22), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
    it('includes the local UTC offset minutes', () => {
      // arrange
      const expectedResult = Math.abs(localNow.getTimezoneOffset() % 60);
      // act
      const result = parseInt(localNowToJSON.substring(23,25), 10);

      // assert
      expect(result).toEqual(expectedResult);
    });
  });
});