Date arithmetic in JavaScript

dates.js

export function addYears(date, numYears) {
    // Make a copy to avoid mutating the date argument
    date = new Date(date.getTime())

    // If adding years to a February 29th ends up in a non-leap year, this
    // returns March 1st, which is the expected result.
    date.setFullYear(date.getFullYear() + numYears)

    // DST can also mess with us. Thanks, JavaScript, for not having dates.
    if (date.getHours() === 23) {
        date.setHours(24)
    } else if (date.getHours() === 1) {
        date.setHours(0)
    }

    return date
}

export function addMonths(date, numMonths) {
    // Make a copy to avoid mutating the date argument
    date = new Date(date.getTime())

    // If adding months to a 29th, 30th, or 31st ends up on a non-existing
    // date, JavaScript tries to be smart and returns the 1st, 2nd or 3rd of
    // the next month. This is too smart for us. We always want the 1st.
    // Track the day number, and if it changes, revert it to 1.
    const oldDay = date.getDate()
    date.setMonth(date.getMonth() + numMonths)
    const newDay = date.getDate()
    if (newDay !== oldDay) {
        date.setDate(1)
    }

    // DST can also mess with us. Thanks, JavaScript, for not having dates.
    if (date.getHours() === 23) {
        date.setHours(24)
    } else if (date.getHours() === 1) {
        date.setHours(0)
    }

    return date
}

test_dates.js

describe("addMonths", () => {

    it("adds months", () => {
        expect(addMonths(new Date('2017-04-21 00:00:00'), 5))
            .toEqual(new Date('2017-09-21 00:00:00'))
        expect(addMonths(new Date('2017-04-21 00:00:00'), 0))
            .toEqual(new Date('2017-04-21 00:00:00'))
        expect(addMonths(new Date('2017-04-21 00:00:00'), -1))
            .toEqual(new Date('2017-03-21 00:00:00'))
    })

    it("handles varying months length", () => {
        expect(addMonths(new Date('2017-01-28 00:00:00'), 1))
            .toEqual(new Date('2017-02-28 00:00:00'))
        expect(addMonths(new Date('2017-01-29 00:00:00'), 1))
            .toEqual(new Date('2017-03-01 00:00:00'))
        expect(addMonths(new Date('2017-01-30 00:00:00'), 1))
            .toEqual(new Date('2017-03-01 00:00:00'))
        expect(addMonths(new Date('2017-01-31 00:00:00'), 1))
            .toEqual(new Date('2017-03-01 00:00:00'))
        expect(addMonths(new Date('2017-02-01 00:00:00'), 1))
            .toEqual(new Date('2017-03-01 00:00:00'))
        expect(addMonths(new Date('2017-02-28 00:00:00'), 1))
            .toEqual(new Date('2017-03-28 00:00:00'))
        expect(addMonths(new Date('2017-03-01 00:00:00'), 1))
            .toEqual(new Date('2017-04-01 00:00:00'))
        expect(addMonths(new Date('2017-03-30 00:00:00'), 1))
            .toEqual(new Date('2017-04-30 00:00:00'))
        expect(addMonths(new Date('2017-03-31 00:00:00'), 1))
            .toEqual(new Date('2017-05-01 00:00:00'))
        expect(addMonths(new Date('2017-08-28 00:00:00'), 6))
            .toEqual(new Date('2018-02-28 00:00:00'))
        expect(addMonths(new Date('2017-08-29 00:00:00'), 6))
            .toEqual(new Date('2018-03-01 00:00:00'))
        expect(addMonths(new Date('2017-08-30 00:00:00'), 6))
            .toEqual(new Date('2018-03-01 00:00:00'))
        expect(addMonths(new Date('2017-08-31 00:00:00'), 6))
            .toEqual(new Date('2018-03-01 00:00:00'))
        expect(addMonths(new Date('2017-09-01 00:00:00'), 6))
            .toEqual(new Date('2018-03-01 00:00:00'))
        expect(addMonths(new Date('2017-09-30 00:00:00'), 6))
            .toEqual(new Date('2018-03-30 00:00:00'))
        expect(addMonths(new Date('2017-10-01 00:00:00'), 6))
            .toEqual(new Date('2018-04-01 00:00:00'))
    })

    it("handles leap years", () => {
        expect(addMonths(new Date('2016-01-29 00:00:00'), 1))
            .toEqual(new Date('2016-02-29 00:00:00'))
        expect(addMonths(new Date('2016-01-30 00:00:00'), 1))
            .toEqual(new Date('2016-03-01 00:00:00'))
        expect(addMonths(new Date('2016-04-29 00:00:00'), -2))
            .toEqual(new Date('2016-02-29 00:00:00'))
        expect(addMonths(new Date('2016-04-30 00:00:00'), -2))
            .toEqual(new Date('2016-03-01 00:00:00'))
    })

    it("handles daylight saving time", () => {
        // This test only validates what it's supposed to validate
        // in the Europe/Paris time zone.
        // CET => CEST
        expect(addMonths(new Date('2017-03-15 00:00:00'), 1))
            .toEqual(new Date('2017-04-15 00:00:00'))
        // CEST => CET
        expect(addMonths(new Date('2017-10-15 00:00:00'), 1))
            .toEqual(new Date('2017-11-15 00:00:00'))
    })

})