C time.h Functions

In this page:

Getting Current Time
Formatting Time as String
Deconstructing Time
Measuring Execution Time
Custom Time Formatting

Getting Current Time

time() returns the current calendar time as the number of seconds elapsed since the Unix epoch (midnight, January 1, 1970 UTC), giving you a single portable integer value you can store, compare, or later convert into a readable date. It's the usual starting point for any time-related calculation in C.

Example: Getting Current Time

#include <stdio.h>
#include <time.h>
int main() {
	time_t now = time(NULL);
	printf("%d", now > 0);
	return 0;
}

Formatting Time as String

ctime() takes the raw time_t value returned by time() and formats it into a fixed, human-readable string containing the day of week, month, date, time, and year, saving you from manually assembling a date string yourself. The returned string already ends in a newline, which is easy to forget when concatenating it elsewhere.

Example: Formatting Time as String

#include <stdio.h>
#include <time.h>
int main() {
	time_t now = time(NULL);
	char *str = ctime(&now);
	printf("%d", str != NULL);
	return 0;
}

Deconstructing Time

localtime() breaks a time_t value down into a struct tm with separate fields for year, month, day, hour, minute, and second, adjusted for the local time zone, so you can inspect or manipulate individual components of a date instead of working with the single opaque seconds-since-epoch number.

Example: Deconstructing Time

#include <stdio.h>
#include <time.h>
int main() {
	time_t now = time(NULL);
	struct tm *local = localtime(&now);
	printf("%d", local->tm_year + 1900);
	return 0;
}

Measuring Execution Time

clock() reports how many CPU clock ticks have elapsed since the program itself started running, which is useful for measuring how long a specific piece of code takes to execute. Dividing the result by CLOCKS_PER_SEC converts that tick count into a number of seconds you can actually reason about.

Example: Measuring Execution Time

#include <stdio.h>
#include <time.h>
int main() {
	clock_t start = clock();
	for (int i = 0; i < 1000; i++) {}
	clock_t end = clock();
	printf("%d", end >= start);
	return 0;
}

Custom Time Formatting

strftime() lets you build a custom date string using format specifiers similar in spirit to printf's, such as %Y for a four-digit year or %H for a 24-hour-clock hour, giving you full control over exactly how a date is displayed instead of being limited to ctime()'s fixed format.

Example: Custom Time Formatting

#include <stdio.h>
#include <time.h>
int main() {
	time_t now = time(NULL);
	struct tm *local = localtime(&now);
	char buffer[20];
	strftime(buffer, 20, "%Y", local);
	printf("%s", buffer);
	return 0;
}

🔒

Chapter Quiz — Complete all 20 topics to unlock

0/20 topics done

Complete these topics first:

← Previous Lesson C math.h Functions Next Lesson → C ctype.h Functions