Comparing two dates

How to:

C doesn’t have a built-in type for dates, necessitating the use of the time.h library to work with date and time structures. The tm structure and difftime() function are commonly used to compare dates. Below is an example showing how to compare two dates:

#include <stdio.h>
#include <time.h>

int main() {
    struct tm date1 = {0};
    struct tm date2 = {0};
    double seconds;

    // First date (YYYY, MM, DD)
    date1.tm_year = 2023 - 1900; // Year since 1900
    date1.tm_mon = 3 - 1;        // Month [0-11]
    date1.tm_mday = 15;          // Day of the month [1-31]

    // Second date (YYYY, MM, DD)
    date2.tm_year = 2023 - 1900;
    date2.tm_mon = 4 - 1;
    date2.tm_mday = 14;

    // Convert to time_t format
    time_t time1 = mktime(&date1);
    time_t time2 = mktime(&date2);

    // Compare
    seconds = difftime(time1, time2);

    if (seconds == 0) {
        printf("Dates are the same.\n");
    } else if (seconds > 0) {
        printf("First date comes after the second date.\n");
    } else {
        printf("First date comes before the second date.\n");

    return 0;

Output could be:

First date comes before the second date.

This program initializes two tm structures with specific dates, converts them to time_t format using mktime(), and finally compares them using difftime(), which returns the difference in seconds (as a double) between the two times.

Deep Dive

In the early days of C, date and time operations required manual calculations, often taking into account leap years, the varying number of days in months, and even leap seconds. The introduction of time.h in the ANSI C standard brought standardization to time handling in C, simplifying date and time operations.

Using time.h for date comparison is straightforward but has limitations. The tm structure does not account for time zones or daylight saving time, and difftime() only provides the difference in seconds, lacking finer granularity for certain applications.

For applications requiring more robust date-time operations, including support for time zones, daylight saving transitions, and more precise time intervals, libraries such as date.h (a Howard Hinnant date library, not part of the standard library) offer a modern alternative to time.h. These libraries provide more comprehensive tools for date-time manipulation in C++, benefiting from decades of evolution in programming language design. For C programmers, leveraging these external libraries or meticulously handling the intricacies of date-time calculations directly remains necessary for achieving precise and culturally aware date-time manipulation.