time.h header file in C with Examples

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. 

1. clock_t: clock_t represents the processor time and is used to measure the CPU clock cycles.
2. time_t: 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. 
3. struct tm: struct tm holds the components of date and time such as hours, minutes, seconds, day, month, year. The struct tm contains:

The time.h header file contains time related operations like getting the current time, converting between different time formats. It also contains CLOCKS_PER_SEC macro which holds the number of times does the system clock ticks per second.

Functions in Time Library

Get Current Date and Time

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

• The time() function retrieves the current time since the beginning of Unix period( 1st Jan, 1970) in the form of time_t value stores it in the variable pointed by the pointer passed to it as an argument.
• The localtime() function to convert the time retrieved by time() function into local time components such as hours, minutes, seconds, day, month and year. It takes one parameter which is a pointer to a time value that is to be converted in local time.
• The asctime() function to convert the obtained time into a string representation of the local time in a human readable format which is "Day Mon Date HH:MM:SS YYYY\n". The asctime() accepts a pointer to a structure (struct tm) that contains the time component that is to be converted.

Example: C program to get current local time

Tue Apr 15 07:22:42 2025

Print Time in UTC (Coordinated Universal Time)

To print the UTC time, we use the gmtime() function that converts the time obtained by the time() function into a Coordinated Universal Time (also known as Greenwich Mean Time).

Tue Apr 15 12:44:17 2025

Find Time Difference

The time difference between the time_t variables recorded between some time intervals is used to check the execution time of the part of the code.

Example: The program uses the difftime() functions defined in the time .h header file. This function take two time value as the starting and ending time and return the difference between them.

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 program uses clock() function defined in time.h which is used to find the number of clock ticks during the executions of a code. Which allows us to measure the time taken by CPU to execute the code.

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

Print Time in hour::minute Format

The program uses strftime() function which formats the date and time as string according to the specified format by the user. To define the format for the time the string uses format specifiers such as %l for hour in 12 hour format, %M for minutes and %p to for AM or PM.