C time.h Library Reference

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    // Method 1: Pass NULL, use return value
6    time_t now = time(NULL);
7    printf("Seconds since Jan 1, 1970: %ld\n", now);
8    
9    // Method 2: Pass pointer to store result
10    time_t current;
11    time(&current);
12    printf("Same value: %ld\n", current);
13    
14    return 0;
15}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    time_t now = time(NULL);
6    
7    // Convert to local time
8    struct tm *local = localtime(&now);
9    printf("Local Time:\n");
10    printf("  Date: %d-%02d-%02d\n", 
11           local->tm_year + 1900,   // Add 1900 to get actual year
12           local->tm_mon + 1,       // Add 1 (0-based month)
13           local->tm_mday);
14    printf("  Time: %02d:%02d:%02d\n",
15           local->tm_hour, local->tm_min, local->tm_sec);
16    
17    // Convert to UTC time
18    struct tm *utc = gmtime(&now);
19    printf("\nUTC Time:\n");
20    printf("  Date: %d-%02d-%02d\n",
21           utc->tm_year + 1900, utc->tm_mon + 1, utc->tm_mday);
22    printf("  Time: %02d:%02d:%02d\n",
23           utc->tm_hour, utc->tm_min, utc->tm_sec);
24    
25    return 0;
26}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    time_t now = time(NULL);
6    
7    // ctime: time_t → string (includes newline)
8    printf("ctime:   %s", ctime(&now));
9    
10    // asctime: struct tm → string
11    struct tm *local = localtime(&now);
12    printf("asctime: %s", asctime(local));
13    
14    // Both produce the same output!
15    return 0;
16}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    time_t now = time(NULL);
6    struct tm *local = localtime(&now);
7    char buffer[100];
8    
9    // Format 1: YYYY-MM-DD
10    strftime(buffer, sizeof(buffer), "%Y-%m-%d", local);
11    printf("ISO Date: %s\n", buffer);
12    
13    // Format 2: DD/MM/YYYY
14    strftime(buffer, sizeof(buffer), "%d/%m/%Y", local);
15    printf("Date: %s\n", buffer);
16    
17    // Format 3: Full date and time
18    strftime(buffer, sizeof(buffer), "%A, %B %d, %Y", local);
19    printf("Full: %s\n", buffer);
20    
21    // Format 4: 12-hour time with AM/PM
22    strftime(buffer, sizeof(buffer), "%I:%M:%S %p", local);
23    printf("Time: %s\n", buffer);
24    
25    // Format 5: 24-hour time
26    strftime(buffer, sizeof(buffer), "%H:%M:%S", local);
27    printf("24h Time: %s\n", buffer);
28    
29    return 0;
30}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    time_t start, end;
6    double elapsed;
7    
8    // Record start time
9    time(&start);
10    
11    // Simulate some work (count to 100 million)
12    printf("Working...\n");
13    long sum = 0;
14    for (long i = 0; i < 100000000; i++) {
15        sum += i;
16    }
17    
18    // Record end time
19    time(&end);
20    
21    // Calculate difference
22    elapsed = difftime(end, start);
23    printf("Elapsed time: %.2f seconds\n", elapsed);
24    
25    return 0;
26}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    struct tm birthday = {0};  // Initialize all fields to 0
6    
7    // Set a specific date: January 26, 2025, 10:30:00
8    birthday.tm_year = 2025 - 1900;  // Years since 1900
9    birthday.tm_mon = 0;             // January (0-based)
10    birthday.tm_mday = 26;           // Day 26
11    birthday.tm_hour = 10;
12    birthday.tm_min = 30;
13    birthday.tm_sec = 0;
14    
15    // Convert to time_t
16    time_t birthdayTime = mktime(&birthday);
17    
18    if (birthdayTime == -1) {
19        printf("Error converting time!\n");
20        return 1;
21    }
22    
23    // Display the created time
24    printf("Birthday: %s", ctime(&birthdayTime));
25    
26    // Calculate days until birthday
27    time_t now = time(NULL);
28    double daysUntil = difftime(birthdayTime, now) / (60 * 60 * 24);
29    printf("Days until birthday: %.0f\n", daysUntil);
30    
31    return 0;
32}

1#include <stdio.h>
2#include <time.h>
3
4int main() {
5    clock_t start, end;
6    double cpu_time;
7    
8    // Record start clock
9    start = clock();
10    
11    // Do some computation
12    printf("Calculating...\n");
13    double result = 0;
14    for (int i = 0; i < 10000000; i++) {
15        result += i * 0.001;
16    }
17    
18    // Record end clock
19    end = clock();
20    
21    // Calculate CPU time in seconds
22    cpu_time = ((double)(end - start)) / CLOCKS_PER_SEC;
23    
24    printf("Result: %.2f\n", result);
25    printf("CPU Time: %.4f seconds\n", cpu_time);
26    printf("Clock ticks: %ld\n", (long)(end - start));
27    
28    return 0;
29}

1#include <stdio.h>
2#include <time.h>
3
4void log_message(const char *level, const char *message) {
5    time_t now = time(NULL);
6    struct tm *local = localtime(&now);
7    char timestamp[20];
8    
9    // Format: [2024-12-15 18:30:45]
10    strftime(timestamp, sizeof(timestamp), "%Y-%m-%d %H:%M:%S", local);
11    
12    printf("[%s] [%s] %s\n", timestamp, level, message);
13}
14
15int main() {
16    log_message("INFO", "Application started");
17    log_message("DEBUG", "Loading configuration");
18    log_message("INFO", "Server listening on port 8080");
19    log_message("WARN", "High memory usage detected");
20    log_message("ERROR", "Failed to connect to database");
21    log_message("INFO", "Application shutting down");
22    
23    return 0;
24}