Standard Library: time.h

The time.h header provides functions for working with dates and times.

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Key Types

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time() Function

Returns the current calendar time as a time_t value (seconds since Jan 1, 1970).

#include <time.h>

time_t now = time(NULL);
printf("Seconds since epoch: %ld\n", (long)now);

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localtime() — Convert to struct tm

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

int main() {
    time_t now = time(NULL);
    struct tm *t = localtime(&now);
    
    printf("Year: %d\n", t->tm_year + 1900);  /* years since 1900 */
    printf("Month: %d\n", t->tm_mon + 1);      /* 0-11 */
    printf("Day: %d\n", t->tm_mday);
    printf("Hour: %d\n", t->tm_hour);
    printf("Minute: %d\n", t->tm_min);
    printf("Second: %d\n", t->tm_sec);
    
    return 0;
}

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struct tm Members

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ctime() — Convert to Readable String

time_t now = time(NULL);
printf("Current time: %s", ctime(&now));
/* Output: Mon Jan 15 10:30:00 2024 */

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clock() — Measure Execution Time

#include <time.h>

clock_t start = clock();

/* ... code to measure ... */
for (int i = 0; i < 1000000; i++);

clock_t end = clock();
double elapsed = (double)(end - start) / CLOCKS_PER_SEC;
printf("Time: %.4f seconds\n", elapsed);

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Using time() to Seed Random Numbers

#include <stdlib.h>
#include <time.h>

srand((unsigned int)time(NULL));
int random = rand() % 100;