Weekly Finder: FreeCodeCamp Daily Challenge

The Challenge

Today’s FreeCodeCamp daily challenge is Weekly Finder. The statement looks simple at first:

Weekday Finder Given a string date in the format YYYY-MM-DD, return the day of the week.

Valid return days are:

• “Sunday”
• “Monday”
• “Tuesday”
• “Wednesday”
• “Thursday”
• “Friday”
• “Saturday”

Be sure to ignore time zones.

At first that last line about “ignore time zones” wasn’t obvious to me, but later I understood why it’s crucial.

First Approach

The initial idea was to create a Date object from the input string and use a formatting method to obtain the weekday name.

I didn’t remember the exact method, but a quick search revealed toLocaleDateString(), which can return the weekday in long form when given the right options. The Date object in JavaScript represents a point in time and can be constructed from an ISO string (YYYY-MM-DD) or from individual components (year, month, day, hour, minute, second). In this challenge we receive an ISO string as input.

toLocaleDateString() formats a Date according to a locale. Passing 'en-US' and { weekday: 'long' } returns the full weekday name (for example: “Monday”, “Tuesday”).

function getDayName(dateString) {
  const date = new Date(dateString)
  return date.toLocaleDateString('en-US', { weekday: 'long' })
}

The function takes a dateString in YYYY-MM-DD format, constructs a Date, and returns the weekday name in English. At first glance, the challenge appeared solved.

The Timezone Problem

But the tests failed. After adding a few console.log statements I discovered the function returned the previous day for every test. Then I remembered the instruction “Be sure to ignore time zones” and realized this was the cause.

TIP

The issue: When JavaScript creates a Date from a string like "2025-11-06", it interprets it as UTC and then converts it to the local timezone — which can change the calendar day.

How to ignore time zones? Very simple: append 'T00:00:00' to the input string. That way we always create the date at midnight local/explicit time and avoid timezone shifts.

function getDayName(dateString) {
  const date = new Date(`${dateString}T00:00:00`)
  return date.toLocaleDateString('en-US', { weekday: 'long' })
}

Alternative Approaches

Option 2: Mathematical Algorithm (Zeller’s Congruence)

Another approach would avoid creating a Date object altogether and compute the weekday using a formula and a weekdays array. It’s more involved and less readable, but perfectly valid.

Zeller’s congruence computes the day of the week for any date:

$f = d + \left\lfloor \frac{13(m+1)}{5} \right\rfloor + y + \left\lfloor \frac{y}{4} \right\rfloor - \left\lfloor \frac{y}{100} \right\rfloor + \left\lfloor \frac{y}{400} \right\rfloor$

Where:

• $d$ = day of month
• $m$ = adjusted month (March=3, …, December=12, January=13, February=14)
• $y$ = year (subtract 1 if month is January or February)

function getDayName(dateString) {
  const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
  const [year, month, day] = dateString.split('-').map(Number)
  const k = month < 3 ? month + 12 : month
  const m = month < 3 ? year - 1 : year
  const f = day + Math.floor((13 * (k + 1)) / 5) + m + Math.floor(m / 4) + Math.floor(m / 400) - Math.floor(m / 100)
  const dayOfWeek = f % 7
  return days[(dayOfWeek + 5) % 7] // Adjustment so 0 = Sunday
}

Comparison of Approaches

This table summarizes pros and cons to help choose the right approach for your needs.

Conclusion

I ended up using the first option because it’s the simplest and most readable. The real challenge was not the algorithm but understanding how JavaScript handles dates and time zones.

NOTE

Lesson learned: When working with dates in JavaScript, always be aware of time zones. A simple T00:00:00 can save you a lot of headaches.