Day of the Year
What is this problem about?
The Day of the Year interview question asks you to calculate the ordinal number of a given date within its year. You are provided with a date string in the format "YYYY-MM-DD", and you need to return an integer representing which day of the year it is. For example, January 1st is the 1st day, and February 1st is the 32nd day. This Day of the Year coding problem is a fundamental exercise in date arithmetic and conditional logic.
Why is this asked in interviews?
Companies like Google and Amazon use this to test a candidate's attention to detail, specifically regarding leap year rules and month length variations. It evaluates your ability to parse strings and correctly accumulate values based on a lookup table (month lengths). It’s a standard "sanity check" problem for entry-level and junior roles.
Algorithmic pattern used
This follows the Math, String interview pattern.
- Parsing: Extract the year, month, and day as integers from the string.
- Pre-calculation: Store the number of days in each of the 12 months in an array.
- Leap Year Check: Adjust the February count if the year is a leap year (divisible by 400 or (divisible by 4 and not by 100)).
- Summation: Sum the days of all full months before the given month, and then add the days of the current month.
Example explanation
Target Date: "2024-03-01"
- Year 2024 is a leap year (2024 % 4 == 0 and 2024 % 100 != 0).
- Month lengths for 2024: [31, 29, 31, 30, ...]
- Full months before March: Jan (31) + Feb (29) = 60.
- Add the current day (1): 60 + 1 = 61.
Result: 61.
Common mistakes candidates make
- Leap Year errors: Forgetting one of the three rules for leap years (the 100 or 400 rules are often missed).
- Zero-indexing confusion: Getting mixed up between month numbers (1-12) and array indices (0-11).
- String Parsing: Using expensive regex when a simple string split or substring is sufficient.
Interview preparation tip
Always remember the leap year rule: (y % 4 == 0 && y % 100 != 0) || (y % 400 == 0). It is a very common requirement in date-related programming tasks.
