# <time.h>

**[<time.h>](https://en.cppreference.com/w/c/header/time)** provides time functions: `time()` for wall-clock seconds since epoch, `clock()` for CPU time, `struct tm` for broken-down calendar time, and `strftime()` for formatting. Use `clock_gettime(CLOCK_MONOTONIC, ...)` for nanosecond-resolution wall-clock timing.

Note: `struct tm` fields like `tm_mon` are 0-indexed.

## Example

This example measures wall-clock and CPU time for a compute loop.

```c
// compile: gcc -o timeexample timeexample.c
// run: ./timeexample
// description: measure CPU and wall-clock time for a loop

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

int main() {
    struct timespec wall_start, wall_end;
    clock_t cpu_start, cpu_end;
    
    clock_gettime(CLOCK_MONOTONIC, &wall_start);
    cpu_start = clock();
    
    volatile long sum = 0;
    for (long i = 0; i < 100000000L; i++)
        sum += i;
    
    cpu_end = clock();
    clock_gettime(CLOCK_MONOTONIC, &wall_end);
    
    double wall = (wall_end.tv_sec - wall_start.tv_sec) +
                  (wall_end.tv_nsec - wall_start.tv_nsec) * 1e-9;
    double cpu = (double)(cpu_end - cpu_start) / CLOCKS_PER_SEC;
    
    printf("wall: %.3f s, cpu: %.3f s\n", wall, cpu);
    return 0;
}
```