The time Library in C: Clocks, Timestamps, and Formatting Dates

0s8 min★ 30 XP

Lesson 99Mixed lesson

The time Library in C: Clocks, Timestamps, and Formatting Dates

A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.

Lesson format15 sections1 video6 code blocks1 practice itemUpdated Aug 27, 2026

The time.h header gives you access to the system clock, calendar time, and CPU time. time(NULL) returns the current Unix timestamp: seconds since January 1, 1970 UTC. clock() returns the CPU time consumed by your process. difftime computes the difference between two calendar times. strftime formats a broken-down time struct into a human-readable string. These four functions cover 90 percent of time-related needs in C programs.

Time Library

#Getting and Formatting the Current Time#link

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

int main(void) {
    time_t now = time(NULL);
    printf("Unix timestamp: %ld\n", (long)now);

    struct tm *local = localtime(&now);
    printf("Local time: %04d-%02d-%02d %02d:%02d:%02d\n",
           local->tm_year + 1900, local->tm_mon + 1, local->tm_mday,
           local->tm_hour, local->tm_min, local->tm_sec);

    char buf[64];
    strftime(buf, sizeof(buf), "%A, %B %d, %Y at %I:%M %p", local);
    printf("Formatted: %s\n", buf);

    return 0;
}

Run time_demo

root@vulnarex:~#gcc -Wall time_demo.c -o time_demo && ./time_demo

tm_year is years since 1900, so add 1900. tm_mon is 0-based (0 = January), so add 1. These offsets are a historical artifact from the 1970s that every C programmer must remember. strftime handles the formatting with printf-like format codes: %Y for four-digit year, %m for zero-padded month, %d for day, %H for 24-hour, %I for 12-hour, %p for AM/PM.

#Measuring Elapsed CPU Time#link

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

long compute_sum(long n) {
    long sum = 0;
    for (long i = 0; i < n; i++) {
        sum += i;
    }
    return sum;
}

int main(void) {
    clock_t start = clock();

    long result = compute_sum(100000000L);

    clock_t end = clock();
    double cpu_seconds = (double)(end - start) / CLOCKS_PER_SEC;

    printf("Sum: %ld\n", result);
    printf("CPU time: %.4f seconds\n", cpu_seconds);

    return 0;
}

Run clock_measure

root@vulnarex:~#gcc -Wall -O2 clock_measure.c -o clock_measure && ./clock_measure

clock() measures CPU time, not wall-clock time. If your process sleeps or waits for I/O, clock does not advance. For wall-clock elapsed time, use time(NULL) before and after, or better, clock_gettime(CLOCK_MONOTONIC) on POSIX for nanosecond resolution. CLOCKS_PER_SEC is typically 1000000 on Linux, giving microsecond granularity.

#Computing Time Differences#link

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

int main(void) {
    struct tm start_tm = {0};
    start_tm.tm_year = 2026 - 1900;
    start_tm.tm_mon = 0;
    start_tm.tm_mday = 1;

    time_t start = mktime(&start_tm);
    time_t now = time(NULL);

    double days = difftime(now, start) / 86400.0;
    printf("Days since Jan 1, 2026: %.1f\n", days);

    return 0;
}

Run time_diff

root@vulnarex:~#gcc -Wall time_diff.c -o time_diff && ./time_diff

★ 30 XP

quiz BLOCK (★ 30 XP)

Why do you add 1900 to tm_year when displaying the year?

Select your proof vectors above

Time handling in C is mechanical but full of off-by-1900 traps. The functions are reliable. The struct fields are quirky. Memorize the offsets and move on. Next: sleep and usleep, for pausing execution with different granularities.

Lesson completion

Ready to resolve this lesson?

Finish the lesson once you have worked through the material. This awards ★ 30 XP.

Previous lesson

Lesson tools

Workspace

0% read

Lab notes

Notes persist per lesson.

Getting and Formatting the Current Time Measuring Elapsed CPU Time Computing Time Differences

Content

Last updated

August 27, 2026

Agent Setup

Access lesson content programmatically for AI agents, LLMs, and automated pipelines.

Fetch as Markdown (Accept header)

curl -H "Accept: text/markdown" "https://vulnarex.com/api/content/lessons?courseSlug=c-programming-tutorials&lessonSlug=99-time-library&lang=en&format=markdown"

MCP Server Config (mcp.json)

{
  "mcpServers": {
    "vulnarex": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-fetch"],
      "env": { "MCP_FETCH_URL": "https://vulnarex.com" }
    }
  }
}

MCP Server Card/.well-known/mcp.json A2A Agent Card/.well-known/agent-card.json API Catalog/.well-known/api-catalog robots.txt/robots.txt

Laboratory sanity code

Isolate active probes on matched virtual networks and keep execution streams sandboxed.

#include <stdio.h>

#include <time.h>

int main(void) {

time_t now = time(NULL);

printf("Unix timestamp: %ld\n", (long)now);

struct tm *local = localtime(&now);

printf("Local time: %04d-%02d-%02d %02d:%02d:%02d\n",

local->tm_year + 1900, local->tm_mon + 1, local->tm_mday,

local->tm_hour, local->tm_min, local->tm_sec);

char buf[64];

strftime(buf, sizeof(buf), "%A, %B %d, %Y at %I:%M %p", local);

printf("Formatted: %s\n", buf);

return 0;

}

#include <stdio.h>

#include <time.h>

long compute_sum(long n) {

long sum = 0;

for (long i = 0; i < n; i++) {

sum += i;

}

return sum;

}

int main(void) {

clock_t start = clock();

long result = compute_sum(100000000L);

clock_t end = clock();

double cpu_seconds = (double)(end - start) / CLOCKS_PER_SEC;

printf("Sum: %ld\n", result);

printf("CPU time: %.4f seconds\n", cpu_seconds);

return 0;

}

#include <stdio.h>

#include <time.h>

int main(void) {

struct tm start_tm = {0};

start_tm.tm_year = 2026 - 1900;

start_tm.tm_mon = 0;

start_tm.tm_mday = 1;

time_t start = mktime(&start_tm);

time_t now = time(NULL);

double days = difftime(now, start) / 86400.0;

printf("Days since Jan 1, 2026: %.1f\n", days);

return 0;

}