Day of the Year in Python

Sometimes we need to find which day of the year a specific date represents. For example, February 10th, 2019 is the 41st day of the year. Python provides several ways to calculate this using date manipulation.

Algorithm Approach

To calculate the day of the year manually, we follow these steps −

• Create an array of days in each month: [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
• Parse the date string to extract year, month, and day
• Check if the year is a leap year and adjust February days to 29
• Sum up all days from months 1 to (month-1) plus the current day

Manual Implementation

class Solution:
    def dayOfYear(self, date):
        # Days in each month (index 0 is placeholder)
        days = [0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
        
        # Parse date string into year, month, day
        year, month, day = map(int, date.split("-"))
        
        # Check for leap year
        if year % 400 == 0 or (year % 4 == 0 and year % 100 != 0):
            days[2] = 29  # February has 29 days in leap year
        
        # Convert to cumulative days
        for i in range(1, len(days)):
            days[i] += days[i-1]
        
        # Return cumulative days up to previous month + current day
        return days[month-1] + day

# Test the solution
solution = Solution()
print(solution.dayOfYear("2019-02-10"))
print(solution.dayOfYear("2020-02-29"))  # Leap year test

41
60

Using Python's datetime Module

Python's built-in datetime module provides a simpler approach using the timetuple() method ?

from datetime import datetime

def day_of_year_datetime(date_string):
    date_obj = datetime.strptime(date_string, "%Y-%m-%d")
    return date_obj.timetuple().tm_yday

# Test with different dates
print(day_of_year_datetime("2019-02-10"))
print(day_of_year_datetime("2020-12-31"))  # Last day of leap year
print(day_of_year_datetime("2021-12-31"))  # Last day of regular year

41
366
365

Using strftime Method

Another concise approach uses the strftime() method with the %j format code ?

from datetime import datetime

def day_of_year_strftime(date_string):
    date_obj = datetime.strptime(date_string, "%Y-%m-%d")
    return int(date_obj.strftime("%j"))

# Test the function
print(day_of_year_strftime("2019-02-10"))
print(day_of_year_strftime("2019-01-01"))  # First day
print(day_of_year_strftime("2019-12-31"))  # Last day

41
1
365

Comparison of Methods

Conclusion

Use datetime.strptime() with strftime("%j") for the simplest solution. The manual approach helps understand leap year calculations and cumulative day counting.