🕐 <time.h> — Date & Time Functions

Provides types and functions to work with calendar time and CPU time. Key types: time_t (seconds since epoch) and struct tm (broken-down calendar time).

Include: #include <time.h> | Compile: gcc -Wall -o prog prog.c

📋 Functions & Macros

Name	Prototype / Value	Description
time	time_t time(time_t *t)	Return current calendar time (Unix epoch)
clock	clock_t clock(void)	Return processor time used by program
difftime	double difftime(time_t t2, time_t t1)	Difference t2 - t1 in seconds
mktime	time_t mktime(struct tm *tm)	Convert struct tm to time_t
gmtime	struct tm gmtime(const time_t t)	Convert time_t to UTC struct tm
localtime	struct tm localtime(const time_t t)	Convert time_t to local struct tm
asctime	char asctime(const struct tm tm)	Convert struct tm to readable string
ctime	char ctime(const time_t t)	Convert time_t to local time string
strftime	size_t strftime(char s, size_t max, const char fmt, const struct tm *tm)	Format time into string

💡 Example Program

Copy, save as demo.c, and compile with the command shown at the bottom.

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

int main(void) {
    time_t now = time(NULL);
    struct tm *lt = localtime(&now);

    char buf[64];
    strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", lt);
    printf("Local time : %s\n", buf);

    const char *days[] = {"Sun","Mon","Tue","Wed","Thu","Fri","Sat"};
    printf("Day        : %s\n", days[lt->tm_wday]);

    clock_t start = clock();
    volatile long sum = 0;
    for (long i = 0; i < 100000000L; i++) sum += i;
    double elapsed = (double)(clock() - start) / CLOCKS_PER_SEC;
    printf("Loop took  : %.4f seconds\n", elapsed);
    return 0;
}
/* Compile: gcc -Wall -O2 -o time_demo time_demo.c */

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