C - time.h in C

1. What is time.h?

time.h is a C standard library header for dealing with date and time.
It provides:

• Types for representing time values

• Functions for getting the current time

• Functions for converting, formatting, and manipulating time

2. Including the Header

#include <time.h>

3. Key Data Types in time.h

4. Important Constants

5. Key Functions in time.h

5.1 Getting the Current Time

time_t t;
time(&t); // store current time in t
printf("%ld\n", t); // seconds since Epoch

Shortcut:

time_t t = time(NULL); // directly returns current time

5.2 Converting to Readable Time

time_t t = time(NULL);
char *str = ctime(&t); // converts to human-readable string
printf("Now: %s", str);

Example output:

Now: Thu Aug 14 10:20:30 2025

5.3 Broken-down Time (struct tm)

You can break a time_t into components:

struct tm *tm_info = localtime(&t);
printf("%d-%02d-%02d %02d:%02d:%02d\n",
       tm_info->tm_year + 1900,  // years since 1900
       tm_info->tm_mon + 1,      // months since Jan (0–11)
       tm_info->tm_mday,
       tm_info->tm_hour,
       tm_info->tm_min,
       tm_info->tm_sec);

5.4 Coordinated Universal Time (UTC)

struct tm *tm_utc = gmtime(&t);

Similar to localtime but gives UTC instead of local time.

5.5 Formatting Time (strftime)

char buffer[80];
strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", tm_info);
printf("Formatted time: %s\n", buffer);

Some common format specifiers:

• %Y → year (e.g., 2025)

• %m → month (01–12)

• %d → day of month

• %H → hour (00–23)

• %M → minute

• %S → second

• %A → weekday name

• %B → month name

5.6 Parsing Time Back (mktime)

Converts a struct tm to time_t:

time_t t2 = mktime(tm_info);

Useful for doing date arithmetic.

5.7 Measuring Elapsed Time

clock_t start = clock();
// some code here
clock_t end = clock();
double cpu_time = (double)(end - start) / CLOCKS_PER_SEC;
printf("Time taken: %.5f seconds\n", cpu_time);

6. Summary Table of Common time.h Functions