Number of Days Between Two Dates - Practice Coding Problems

Number of Days Between Two Dates - Problem

Math String Easy

Write a program to count the number of days between two dates.

The two dates are given as strings in the format YYYY-MM-DD. You need to return the absolute difference in days between the two dates.

Note: The dates are valid and within the range of years 1971 to 2100.

Input & Output

Example 1 — Basic Case

$ Input: date1 = "2019-06-29", date2 = "2019-09-30"

› Output: 93

💡 Note: From June 29 to September 30: 1 day in June + 31 days in July + 31 days in August + 30 days in September = 93 days

Example 2 — Same Year

$ Input: date1 = "2020-01-15", date2 = "2020-03-18"

› Output: 63

💡 Note: From January 15 to March 18: 16 days in January + 29 days in February (2020 is leap year) + 18 days in March = 63 days

Example 3 — Same Date

$ Input: date1 = "2019-12-25", date2 = "2019-12-25"

› Output: 0

💡 Note: Both dates are identical, so the difference is 0 days

Constraints

The dates are valid and within the range of years 1971 to 2100
Date format is always YYYY-MM-DD
Both dates will have valid day and month values

Visualization

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Understanding the Visualization

Input

Two dates in YYYY-MM-DD format

Process

Convert to common reference and calculate difference

Output

Absolute number of days between dates

Key Takeaway

🎯 Key Insight: Convert both dates to days since a common reference point, then simple subtraction gives the answer efficiently

Asked in

G Google 15 M Microsoft 12 A Amazon 8 F Facebook 6

The key insight is converting dates to a common reference point (days since epoch) for easy arithmetic. The optimal approach uses mathematical calculation to convert each date to days since January 1, 1970, then finds the absolute difference. Time: O(1), Space: O(1).

Common Approaches

Approach	Time	Space	Notes
✓ Day-by-Day Counting	O(d)	O(1)	Count each day incrementally from one date to another
Days Since Epoch	O(1)	O(1)	Convert each date to days since a reference point, then subtract

Day-by-Day Counting — Algorithm Steps

Parse both dates into year, month, day components
Determine which date is earlier
Count days incrementally, handling month/year transitions

Visualization

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Step-by-Step Walkthrough

Parse Dates

Extract year, month, day from both date strings

Count Days

Increment day by day, handling month/year boundaries

Result

Total days counted between the two dates

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int isLeapYear(int year) {
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
}

int daysInMonth(int year, int month) {
    int days[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    if (month == 2 && isLeapYear(year)) {
        return 29;
    }
    return days[month - 1];
}

void parseDate(char* date, int* year, int* month, int* day) {
    sscanf(date, "%d-%d-%d", year, month, day);
}

int solution(char* date1, char* date2) {
    int y1, m1, d1, y2, m2, d2;
    parseDate(date1, &y1, &m1, &d1);
    parseDate(date2, &y2, &m2, &d2);
    
    // Ensure date1 is earlier
    if (y1 > y2 || (y1 == y2 && m1 > m2) || (y1 == y2 && m1 == m2 && d1 > d2)) {
        int ty = y1, tm = m1, td = d1;
        y1 = y2; m1 = m2; d1 = d2;
        y2 = ty; m2 = tm; d2 = td;
    }
    
    int days = 0;
    while (y1 != y2 || m1 != m2 || d1 != d2) {
        days++;
        d1++;
        if (d1 > daysInMonth(y1, m1)) {
            d1 = 1;
            m1++;
            if (m1 > 12) {
                m1 = 1;
                y1++;
            }
        }
    }
    
    return days;
}

int main() {
    char date1[12], date2[12];
    fgets(date1, sizeof(date1), stdin);
    fgets(date2, sizeof(date2), stdin);
    
    // Remove newlines
    date1[strcspn(date1, "\n")] = 0;
    date2[strcspn(date2, "\n")] = 0;
    
    int result = solution(date1, date2);
    printf("%d\n", result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(d)

Where d is the number of days between dates - we iterate through each day

✓ Linear Growth

Space Complexity

O(1)

Only using variables to track current date components

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Day-by-Day Counting — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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