# <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;
}
```

## Common functions

**Current time** (seconds since epoch, 1970-01-01 00:00:00 UTC):
- `time(&t)`: get current wall-clock time (returns time_t, sets t if non-NULL)
- `clock()`: get CPU time used by process (call twice and subtract to measure)

**High-resolution time** (POSIX, not standard C):
- `clock_gettime(clock_id, &timespec)`: high-resolution time with nanoseconds
- `CLOCK_REALTIME`: wall-clock time
- `CLOCK_MONOTONIC`: monotonic time (unaffected by system clock adjustments)

**Time conversions** (to/from struct tm):
- `localtime(&time_t)`: convert to local time struct (uses system timezone)
- `gmtime(&time_t)`: convert to UTC time struct
- `mktime(&tm)`: convert local time struct to time_t
- `localtime_r(&time_t, &tm)`: reentrant version of localtime (POSIX)
- `gmtime_r(&time_t, &tm)`: reentrant version of gmtime (POSIX)

**Formatting time to string**:
- `strftime(buf, size, fmt, &tm)`: format struct tm using format string
- `asctime(&tm)`: format as "Dow Mon DD HH:MM:SS YYYY\n" (deprecated)
- `ctime(&time_t)`: shorthand for asctime(localtime(t)) (deprecated)

**Parsing time from string** (POSIX):
- `strptime(str, fmt, &tm)`: parse string into struct tm

**Time arithmetic**:
- `difftime(t1, t2)`: seconds between t1 and t2

**Types and constants**:
- `time_t`: time value (seconds since epoch)
- `struct tm`: broken-down time (year, month, day, hour, minute, second, etc.)
- `struct timespec`: time with nanosecond precision (tv_sec, tv_nsec)
- `CLOCKS_PER_SEC`: conversion factor for clock() output to seconds