Dan Levy

Quiz: 14 JavaScript Date Questions

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How well do you know the Date class?

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  • Multiple choice. 🤖 … How difficult can it be, eh?

Outline

The Date class in JavaScript has a notoriously difficult API. It was inherited from Java, and I can only assume was inspired ancient neolithic time keeping methods.

The struggle to get Date’s leads many developers to use 3rd party libraries without question. While often a safe and reliable choice, these libraries are rarely needed for formatting dates or localization!

This quiz is designed to test (and deepen) your knowledge of the native Date API. Use the green buttons for hints & explanations! Hopefully by the end of the challenge you’ll have solidified your understanding of Date’s in JavaScript.

NOTE: Assume all examples assume GMT-7 local timezone.

👇 14 Questions Below 👇

1. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

const d1 = new Date(2020, 1, 1)
console.log(d1)

The Month argument is zero-based. With a range of 0-11 (using western calendars.)

‘February’ has an index value of one. (Think of it like an array lookup.)

2. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

const d2 = new Date(2020, 0, 1)
console.log(d2)

The Month argument is zero-based. With a range of 0-11 (using western calendars.)

‘January’ has an index value of zero. (Think of it like an array lookup.)

3. 

Handling Dates

What will the output include?

const d3 = Date('Thu, 01 Jan 1970 00:00:00 GMT')
console.log(d3)

Calling Date() without new ignores all arguments and returns a string for the current date and time. Use new Date(...) to create a Date object from the supplied arguments.

4. 

Handling Dates

Assume the local timezone is fixed at GMT−7.

What will the output include?

const date = new Date(2020)
console.log(date.getFullYear())

A Date instance created with a single integer argument is interpreted as a Unix Epoch value. Epoch is a count of milliseconds since Jan 1st, 1970.

A value of 2020 (milliseconds) translates to 2 seconds after Jan 1st, 1970.

Then since our local time zone is a negative offset of -7 hours, we end up with Wed Dec 31 1969 17:00:02 GMT-0700 (Mountain Standard Time).

You can side-step the local timezone offset by using .getUTCFullYear().

5. 

Handling Dates

Assume the local timezone is fixed at GMT−7.

What value will print to the console?

const d1 = new Date('2020-01-01')
const d2 = new Date('2020-01-01T00:00')
console.log(d1.getFullYear(), d2.getFullYear())

The string without a T time value may appear to be Jan 1st, 2020 - but date-only strings are interpreted as UTC, and when adjusted to our local timezone (GMT-7) we find we’re still in 2019.

Date-time strings without an explicit timezone are interpreted in local time.

The T00:00 form causes the second value to be interpreted as local midnight.

The first date is interpreted as Tue Dec 31 2019 17:00:00 GMT-0700 (Mountain Standard Time). The second date is interpreted as Wed Jan 01 2020 00:00:00 GMT-0700 (Mountain Standard Time).

6. 

Handling Dates

Select an incorrect formatting method:

The method toLocaleFormat() is not part of the ECMAScript standard. Use a standard formatting method such as toLocaleDateString() or toLocaleString() for portable code.

Check out the toLocaleDateString docs method. It’s behavior is documented under Intl.DateTimeFormat.

7. 

Handling Dates

What will the output include?

var date = Date.UTC('2020-01-02T00:00')
console.log(date.toUTCString())

Date.UTC() expects numeric year/month/etc. arguments. This string cannot be converted to a valid year, so the call first returns NaN. That is a number, not a Date object; calling its .toUTCString() throws TypeError.

8. 

Handling Dates

What will the output include?

const d = Date.UTC(2020, 0, 1)
console.log(d)

The helper method Date.UTC doesn’t return a date instance. It returns an integer in milliseconds.

9. 

Handling Dates

What will the output include?

// Assume local TZ is -07:00
const d = new Date(Date.UTC(2020, 0, 1))
console.log(d.getTimezoneOffset())

Date objects store milliseconds since the Unix epoch, not a timezone. getTimezoneOffset() uses the local timezone rules for that date and returns the difference from UTC in minutes. At the stated GMT−7 offset, the result is 420. Offsets can differ across dates in regions that observe daylight saving time.

10. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

  const d = new Date(2020, 0, 1)
  d.setDate(1)
  console.log(d)

The .setDate() method sets the day of the month, based on the given instance’s current month.

If a value is provided outside of the number of days available, the date instance month value will be adjusted (e.g. A setDate(32) in January will calculate as February 1st.)

11. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

  const d = new Date(2020, 0, 1)
  d.setMonth(1)
  console.log(d)

The .setMonth() method sets the month of the given date instance.

The month argument is zero-based, with a range of 0-11 (using western calendars.)

12. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

  const d = new Date(2020, 0, 1)
  d.setMonth(12)
  console.log(d)

The .setMonth() method sets the month of the given date instance.

The month argument is zero-based, with 12 values in the range of 0-11 (using western calendars.)

Here we see the year is adjusted to 2021, because setMonth(12) is 1 more than 11 (December).

13. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

  const d = new Date(2020, 0, 1)
  d.setMonth(13)
  console.log(d)

The .setMonth() method sets the month of the given date instance.

The month argument is zero-based, with a range of 0-11 (using western calendars.)

Here we see the month and year is adjusted to February 2021, because setMonth(13) is 2 more than 11 (December).

14. 

Handling Dates

Which local calendar date does this object represent? Console display formatting may vary.

  const d = new Date(2020, 0, 1)
  d.setMonth(-1)
  console.log(d)

The .setMonth() method sets the month of the given date instance.

The month argument is zero-based, with a range of 0-11 (using western calendars.)

Here we see the month and year rolls back to December 2019, because setMonth(-1) is less than 0 (January).

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