How would you improve this code?

day-spec.js

import {
  assertThat,
  equalTo,
} from 'hamjest';
import { getDaysInMonth } from './day';

describe('getDaysInMonth()', () => {
  it('April has 30 days', () =>
    assertThat(getDaysInMonth(4, 2016), equalTo(30)));

  it('June has 30 days', () =>
    assertThat(getDaysInMonth(6, 2016), equalTo(30)));

  it('Sept has 30 days', () =>
    assertThat(getDaysInMonth(9, 2016), equalTo(30)));

  it('Nov has 30 days', () =>
    assertThat(getDaysInMonth(11, 2016), equalTo(30)));

  describe('leap years', () => {
    it('Feb in 1900 has 28 days', () =>
      assertThat(getDaysInMonth(2, 1900), equalTo(28)));

    it('Feb in 1994 has 28 days', () =>
      assertThat(getDaysInMonth(2, 1994), equalTo(28)));

    it('Feb in 1996 has 29 days', () =>
      assertThat(getDaysInMonth(2, 1996), equalTo(29)));

    it('Feb in 1997 has 28 days', () =>
      assertThat(getDaysInMonth(2, 1997), equalTo(28)));

    it('Feb in 2000 has 29 days', () =>
      assertThat(getDaysInMonth(2, 2000), equalTo(29)));

    it('Feb in 2016 has 29 days', () =>
      assertThat(getDaysInMonth(2, 2016), equalTo(29)));

    it('Feb in 2400 has 29 days', () =>
      assertThat(getDaysInMonth(2, 2400), equalTo(29)));

    it('Feb in 2401 has 28 days', () =>
      assertThat(getDaysInMonth(2, 2401), equalTo(28)));
  });
});

day.js

export const getDaysInMonth = (month, year) => {
  const isFebruary = month === 2;
  const isLeapYear = (!(year % 4) && year % 100) || !(year % 400);

  if(isFebruary) {
    return isLeapYear ? 29 : 28;
  }

  const SEPTEMBER = 9;
  const APRIL = 4;
  const JUNE = 6;
  const NOVEMBER = 11;

  const is30DaysMonth = new RegExp(`${SEPTEMBER}|${APRIL}|${JUNE}|${NOVEMBER}`).test(month);
  return is30DaysMonth ? 30 : 31;
};