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Next Day - Solution & Explanation

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Problem Statement

Write code that enhances all date objects such that you can call the date.nextDay() method on any date object and it will return the next day in the format YYYY-MM-DD as a string.

 

Example 1:

Input: date = "2014-06-20"
Output: "2014-06-21"
Explanation: 
const date = new Date("2014-06-20");
date.nextDay(); // "2014-06-21"

Example 2:

Input: date = "2017-10-31"
Output: "2017-11-01"
Explanation: The day after 2017-10-31 is 2017-11-01.

 

Constraints:

  • new Date(date) is a valid date object

Approach Overview

Problem Overview: Given a date in YYYY-MM-DD format, compute the calendar date for the next day. The solution must correctly handle month boundaries, leap years, and year transitions such as 2023-12-31 → 2024-01-01.

Approach 1: Built-in Date Handling (O(1) time, O(1) space)

The simplest way is to rely on the language’s built-in date utilities. Parse the input string into a Date object, add one day using a timestamp or date method, then format the result back to YYYY-MM-DD. Internally the runtime already handles leap years, varying month lengths, and year rollovers. This approach is concise and reliable for production code. The key operations are parsing the string, incrementing the day value, and formatting the output string. While the underlying library performs more work internally, from an algorithmic perspective the operations run in constant time O(1) and use O(1) space.

Approach 2: Manual Calendar Simulation (O(1) time, O(1) space)

This approach parses the year, month, and day integers and manually simulates calendar rules. Maintain an array of days per month and adjust February when the year is a leap year. A leap year occurs when (year % 4 == 0 && year % 100 != 0) || (year % 400 == 0). Increment the day by one, then check if it exceeds the maximum days for that month. If it does, reset the day to 1 and increment the month. If the month becomes 13, reset it to 1 and increment the year. Finally, format the result with leading zeros to maintain the YYYY-MM-DD format. This solution is a straightforward simulation problem using basic math conditions and simple string formatting.

Recommended for interviews: Manual calendar simulation. Interviewers want to see that you understand leap year rules and boundary transitions between days, months, and years. The built-in date approach demonstrates practical engineering, but the simulation approach shows stronger problem-solving ability and control over edge cases.

Solution

Code

TypeScript

TypeScript

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Detailed Complexity Analysis

ApproachTimeSpaceWhen to Use
Built-in Date HandlingO(1)O(1)Quick implementation in real-world code where standard libraries are allowed
Manual Calendar SimulationO(1)O(1)Preferred in interviews to demonstrate understanding of leap years and date boundaries

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Frequently Asked Questions

Is Next Day easy or hard?
Next Day is classified as an Easy problem. The challenge mainly comes from handling edge cases such as February in leap years and transitions like the end of a month or year.
Next Day Python/Java solution
In Python or Java, the problem can be solved either with built-in date libraries or with manual simulation. The manual approach parses the date string, checks leap years, updates day and month boundaries, and formats the output back to YYYY-MM-DD in O(1) time.
How to solve Next Day in O(1)?
Parse the date components, increment the day, and compare it with the maximum days in the current month. If the day exceeds the limit, reset it to 1 and increment the month; if the month exceeds 12, reset it to 1 and increment the year. Leap year rules adjust February to 29 days when needed.
What is the best approach for Next Day?
Manual calendar simulation is the most common approach. Parse the year, month, and day, increment the day, and adjust for month length and leap years. The algorithm runs in O(1) time and O(1) space because it only performs fixed arithmetic checks and formatting.
Is Next Day asked at Google/Amazon/Meta?
Date manipulation problems appear frequently in interviews across large tech companies because they test edge case handling and careful logic. Variants involving leap years, date differences, or calendar simulations are commonly used in screening rounds.
What data structure is used in Next Day?
Most implementations use a small array storing the number of days in each month. Combined with integer variables for year, month, and day, this allows constant-time checks and updates during the simulation.
What is the time complexity of Next Day?
The time complexity is O(1). The algorithm performs a constant number of operations: parsing the date, checking leap year conditions, updating day/month/year values, and formatting the result string.

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