time.h header file in C with Examples

Last Updated : 12 Aug, 2026

The time.h header file contains definitions of functions to get and manipulate date and time information. It also includes functions, types and macros, which are used by programs to measure time, manipulate dates and format time information. It describes three time-related data types. 

  • clock_t represents the processor time and is used to measure the CPU clock cycles.
  • time_t represents the clock time as an integer which shows the number of seconds since the beginning of the Unix Epoch ( 1st January 1970, UTC), which is a part of the calendar time. 
  • struct tm holds the components of date and time such as hours, minutes, seconds, day, month, year. The struct tm contains:
C 
struct tm {
    // seconds,  range 0 to 59
    int tm_sec;

    // minutes, range 0 to 59
    int tm_min;

    // hours, range 0 to 23
    int tm_hour;

    // day of the month, range 1 to 31
    int tm_mday;

    // month, range 0 to 11
    int tm_mon;

    // The number of years since 1900
    int tm_year;

    // day of the week, range 0 to 6
    int tm_wday;

    // day in the year, range 0 to 365
    int tm_yday;

    // daylight saving time
    int tm_isdst;
}

Explanation:

  • The time.h header file provides functions for handling date and time operations in C.
  • It is used to get the current time.
  • It provides functions to convert between different time formats.
  • It contains the CLOCKS_PER_SEC macro, which specifies the number of clock ticks per second.

Get Current Date and Time

To get the current date and time we use the combination of time() , localtime(), and asctime() functions as shown:

  • time(): Gets the current time since January 1, 1970 and stores it as a time_t value.
  • localtime(): Converts time_t into local time components such as hour, minute, day, month, and year.
  • asctime(): Converts the time components into a human-readable string format.
C 
#include <stdio.h>
#include <time.h>

int main() {
    
    // Structure to store local time
    struct tm* ptr;
    
    // Variable to store current time
    time_t t;
    
    // Get current time
    t = time(NULL);
    
    // Convert it to local time
    ptr = localtime(&t);
    
    // Get the string of local time
    printf("%s", asctime(ptr));
    return 0;
}

Output
Tue Apr 15 07:22:42 2025

The gmtime() function converts the time obtained using time() into Coordinated Universal Time (UTC).

  • It is used to display the current time in UTC format.
  • UTC is also commonly referred to as Greenwich Mean Time (GMT).
C 
#include <stdio.h>
#include <time.h>

int main() {
    
    // Structure to store local time
    struct tm* ptr;
    
    // Variable to store current time
    time_t t;
    
    // Get current time
    t = time(NULL);
    
    // Convert it to UTC time
    ptr = gmtime(&t);
    
    // Get the string of local time
    printf("%s", asctime(ptr));
    return 0;
}

Output
Tue Apr 15 12:44:17 2025

Find Time Difference

The difference between two time_t values recorded at different time intervals can be used to measure the execution time of a part of the code.

  • Subtract the earlier time_t value from the later value to find the time difference.
  • It helps determine how much time a specific part of the code takes to execute.
C 
#include <stdio.h>
#include <time.h>

int main() {
    time_t start, end;
    
    // Record start time
    start = time(NULL);
    int a, b;
    scanf("%d %d", &a, &b);
    printf("Sum of %d and %d is %d\n",
           a, b, a + b);
           
    // Record endtime
    end = time(NULL);
    
    // Print time difference
    printf("Time taken to print sum is %.2f seconds",
           difftime(end, start));
}

Output
Sum of 0 and 0 is 0
Time taken to print sum is 0.00 seconds

Note: If user gives input slowly that time also add up for total execution time.

Time using CPU Clock

The clock() function from time.h is used to measure the CPU time taken by a program during execution.

  • It calculates the number of clock ticks used during execution.
  • The elapsed CPU time can be calculated using CLOCKS_PER_SEC.
C 
#include <math.h>
#include <stdio.h>
#include <time.h>

int frequency_of_primes(int n)
{
    // This function checks the number of
    // primes less than the given parameter
    int i, j;
    int freq = n - 1;
    for (i = 2; i <= n; ++i)
        for (j = sqrt(i); j > 1; --j)
            if (i % j == 0) {
                --freq;
                break;
            }
    return freq;
}

int main()
{
    clock_t t;
    int f;
    t = clock();
    f = frequency_of_primes(9999);
    printf("The number of primes lower"
           " than 10, 000 is: %d\n",
           f);
    t = clock() - t;
    printf("No. of clicks %ld clicks (%f seconds).\n",
           t, ((float)t) / CLOCKS_PER_SEC);
    return 0;
}

Output
The number of primes lower than 10, 000 is: 1229
No. of clicks 2837 clicks (0.002837 seconds).

The strftime() function formats date and time into a string according to the specified format.

  • %I represents the hour in 12-hour format.
  • %M represents minutes, and %p represents AM or PM.
C 
#include <stdio.h>
#include <time.h>
int main()
{
    time_t rawtime;
    struct tm* timeinfo;

    // Used to store the time
    // returned by localtime() function
    char buffer[80];

    time(&rawtime);
    timeinfo = localtime(&rawtime);
    strftime(buffer, 80,
             "Time is %I:%M %p.",
             timeinfo);

    // strftime() function stores the
    // current time as Hours : Minutes
    //%I %M and %p-> format specifier
    // of Hours minutes and am/pm respectively*/

    // prints the formatted time
    puts(buffer);

    return 0;
}

Output
Time is 09:00AM.

Functions in Time Library

Function NameExplanation
asctime()This function returns the date and time in the format 
day month date hours:minutes:seconds year.
Eg: Sat Jul 27 11:26:03 2019.
asctime() function returns a string by taking struct tm variable as a parameter.
clock()This function returns the processor time consumed by a program
ctime()This function returns the date and time in the format
day month date hours:minutes:seconds year
Eg: Sat Jul 27 11:26:03 2019
time is printed based on the pointer returned by Calendar Time
difftime()This function returns the difference between the times provided.
gmtime()This function prints the UTC (Coordinated Universal Time) Time and date.
Format for both gmtime() and asctime() is same
mktime()This function returns the calendar-time equivalent using struct tm.
time()This function returns the calendar-time equivalent using data-type time_t.
strftime()This function helps to format the string returned by other time functions using different format specifiers
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