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Day of the Year - Solution & Explanation

EasyMathString14 min readAsked at: Meta, Zscaler, Google
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Problem Statement

Given a string date representing a Gregorian calendar date formatted as YYYY-MM-DD, return the day number of the year.

 

Example 1:

Input: date = "2019-01-09"
Output: 9
Explanation: Given date is the 9th day of the year in 2019.

Example 2:

Input: date = "2019-02-10"
Output: 41

 

Constraints:

  • date.length == 10
  • date[4] == date[7] == '-', and all other date[i]'s are digits
  • date represents a calendar date between Jan 1st, 1900 and Dec 31st, 2019.

Approach Overview

Problem Overview: You receive a date string in the format YYYY-MM-DD. The task is to return which day of the year that date represents. For example, 2019-02-10 corresponds to day 41. The main challenge is correctly handling month lengths and leap years.

Approach 1: Using Predefined Days in Months (O(1) time, O(1) space)

This approach manually calculates the day number by summing the number of days in all months before the given month, then adding the current day. Start by parsing the input string to extract year, month, and day. Maintain an array like [31,28,31,30,31,30,31,31,30,31,30,31] representing days in each month. If the year is a leap year (divisible by 400, or divisible by 4 but not 100), update February to 29 days. Iterate through months before the current month and accumulate their days, then add the given day. The algorithm performs a small fixed loop of at most 12 iterations, so the time complexity is effectively O(1) with O(1) extra space.

This method relies on basic math logic for leap year detection and simple parsing using string operations. Because it avoids heavy libraries and clearly demonstrates the leap year rule, it is the most common implementation used in interviews.

Approach 2: Using Date Libraries for Direct Calculation (O(1) time, O(1) space)

Many languages provide built-in date libraries that already handle calendar calculations. Instead of manually counting days, convert the input string into a date object and compute the difference between that date and the first day of the same year (YYYY-01-01). The difference in days plus one gives the day-of-year value. In JavaScript, for example, you can construct Date objects and subtract timestamps to obtain the day count.

This approach delegates leap year handling, month lengths, and calendar rules to the standard library. It keeps the implementation short and reduces the chance of logical errors. The complexity remains O(1) because all operations are constant-time calculations performed by the library.

Recommended for interviews: The predefined-days approach is typically expected. It shows that you understand leap year rules and can implement simple calendar math without relying on built-in utilities. The date-library solution is perfectly valid in production code, but interviewers often prefer seeing the manual calculation because it demonstrates reasoning with arrays, conditions, and basic math logic.

Approach 1: Using Predefined Days in Months

This approach involves creating a static list of the total days for each month. Parse the given date to extract the year, month, and day. Check if the year is a leap year to adjust February's days if necessary. Sum the days from previous months and add the current month's day to find the day of the year.

The function isLeapYear checks if a year is a leap year. The dayOfYear function parses the date string, then computes the day of the year by summing month days and adjusts if it's a leap year.

Code

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Complexity

Time Complexity: O(1) because the number of months is constant.
Space Complexity: O(1) because we're using a fixed-size array and a few integers.

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Approach 2: Using Date Libraries for Direct Calculation

This approach utilizes built-in or third-party date libraries (where applicable) to parse the input string and directly acquire the day of the year. This reduces manual effort by leveraging efficient, existing functions.

The code snippet uses JavaScript Date to parse and compute the day of the year by calculating the difference in days from the start of the year.

Code

JavaScript

JavaScript

Complexity

Time Complexity: O(1).
Space Complexity: O(1).

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Approach 3: Direct Calculation

According to the problem, the given date is in the Gregorian calendar, so we can directly calculate which day of the year it is.

First, calculate the year, month, and day from the given date, denoted as y, m, d.

Then, calculate the number of days in February of that year according to the leap year rules of the Gregorian calendar. There are 29 days in February of a leap year and 28 days in a non-leap year.

The leap year calculation rule is: the year can be divided by 400, or the year can be divided by 4 but not by 100.

Finally, calculate which day of the year it is according to the given date, that is, add up the number of days in each previous month, and then add the number of days in the current month.

The time complexity is O(1), and the space complexity is O(1).

Code

PythonJavaC++GoTypeScriptJavaScript

Python

Java

C++

Go

TypeScript

JavaScript

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Complexity Comparison

ApproachComplexity
Using Predefined Days in Months

Time Complexity: O(1) because the number of months is constant.
Space Complexity: O(1) because we're using a fixed-size array and a few integers.

Using Date Libraries for Direct Calculation

Time Complexity: O(1).
Space Complexity: O(1).

Direct Calculation—

Detailed Complexity Analysis

ApproachTimeSpaceWhen to Use
Using Predefined Days in MonthsO(1)O(1)Best for interviews when implementing leap year logic manually
Using Date LibrariesO(1)O(1)Useful in production code where built-in date utilities are available

Video Solution

1154. Day of the Year (Leetcode Easy) • Programming Live with Larry • 946 views views

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Frequently Asked Questions

Is Day of the Year easy or hard?
Day of the Year is classified as an Easy problem. The main concepts are string parsing, simple arithmetic, and applying the leap year rule correctly.
How to solve Day of the Year in O(1)?
Extract the year, month, and day from the YYYY-MM-DD string. Use a fixed array of month lengths, adjust February to 29 if the year is a leap year, sum the days of months before the given month, and add the day value. Since the loop iterates over at most 12 months, the runtime is constant.
What is the best approach for Day of the Year?
The predefined days-per-month approach is the most common solution. Parse the date, sum the days from previous months, adjust February for leap years, and add the current day. This runs in O(1) time and O(1) space because the number of months is fixed.
What data structure is used in Day of the Year?
Most solutions use a small array (or list) storing the number of days in each month. The algorithm also relies on basic string parsing and conditional logic for leap year detection.
What is the time complexity of Day of the Year?
The standard solution runs in O(1) time and O(1) space. At most 12 months are processed when summing days, which is a constant bound independent of input size.
Day of the Year Python or Java solution approach?
In Python or Java, split the date string to extract year, month, and day, store month lengths in an array, adjust February for leap years, and sum previous months. The implementation is short and runs in constant time.
Is Day of the Year asked at Google, Amazon, or Meta?
Problems involving date parsing, leap year rules, and simple calendar calculations appear in coding interviews across companies like Amazon and Google. While this exact question may not always appear, similar math and string manipulation problems are common.

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