Time Functions in C – time(), ctime(), localtime(), strftime()

The time.h library in C provides functions for getting the current date and time, formatting it for display, and converting between representations. The central type is time_t — typically a 32-bit or 64-bit integer counting seconds since the Unix epoch (1970-01-01 00:00:00 UTC). Five key functions cover the most common time operations: time(), ctime(), localtime(), asctime(), and strftime().

time.h Functions at a Glance

Function	Input	Output	Use case
`time(NULL)`	nothing	`time_t` (epoch seconds)	Get current time as a number
`ctime(&t)`	`time_t *`	`char *` with newline	Quick human-readable timestamp
`localtime(&t)`	`time_t *`	`struct tm *`	Get year/month/day/hour/min/sec separately
`asctime(tm)`	`struct tm *`	`char *` with newline	Format a struct tm like ctime
`strftime(buf, n, fmt, tm)`	`struct tm *`	written to `buf`	Custom date/time format string

C Program for Time Functions

/* time.h functions: time(), ctime(), localtime(), asctime(), strftime()
 * Compile: gcc -ansi -Wall -Wextra timefuncs.c -o timefuncs */
#include <stdio.h>
#include <time.h>

int main(void)
{
    time_t now;
    struct tm *local;
    char buf[80];

    /* time() returns seconds since the Unix epoch (1970-01-01 00:00:00 UTC) */
    now = time(NULL);
    printf("Seconds since epoch : %ld\n\n", (long)now);

    /* ctime() formats the time_t value as a human-readable string */
    printf("ctime()             : %s", ctime(&now));  /* ctime adds '\n' */

    /* localtime() converts time_t to a broken-down local-time struct tm */
    local = localtime(&now);
    printf("localtime fields:\n");
    printf("  Year : %d\n", local->tm_year + 1900);  /* years since 1900 */
    printf("  Month: %d\n", local->tm_mon + 1);       /* 0-based, so +1 */
    printf("  Day  : %d\n", local->tm_mday);
    printf("  Hour : %d\n", local->tm_hour);
    printf("  Min  : %d\n", local->tm_min);
    printf("  Sec  : %d\n\n", local->tm_sec);

    /* asctime() formats a struct tm to a string (same format as ctime) */
    printf("asctime()           : %s", asctime(local));

    /* strftime() formats with full control over the output string */
    strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S", local);
    printf("strftime ISO 8601   : %s\n", buf);

    strftime(buf, sizeof(buf), "%A, %B %d, %Y", local);
    printf("strftime verbose    : %s\n", buf);

    return 0;
}

How to Compile and Run

gcc -ansi -Wall -Wextra timefuncs.c -o timefuncs
./timefuncs

Sample Output

Seconds since epoch : 1782751735

ctime()             : Mon Jun 29 16:48:55 2026
localtime fields:
  Year : 2026
  Month: 6
  Day  : 29
  Hour : 16
  Min  : 48
  Sec  : 55

asctime()           : Mon Jun 29 16:48:55 2026
strftime ISO 8601   : 2026-06-29 16:48:55
strftime verbose    : Monday, June 29, 2026

struct tm Fields Reference

Field	Type	Range	Note
`tm_year`	int	years since 1900	Add 1900 to get the calendar year
`tm_mon`	int	0–11	0=January, 11=December — add 1 for human display
`tm_mday`	int	1–31	Day of month (1-based, unlike tm_mon)
`tm_hour`	int	0–23	24-hour format
`tm_min`	int	0–59	Minutes
`tm_sec`	int	0–60	60 allows for a leap second
`tm_wday`	int	0–6	0=Sunday, 6=Saturday
`tm_yday`	int	0–365	Day of year (0=Jan 1)
`tm_isdst`	int	positive/0/−1	Daylight saving: >0=yes, 0=no, −1=unknown

strftime Format Codes

Code	Example	Description
%Y	2026	4-digit year
%m	06	Month 01–12
%d	29	Day 01–31
%H	16	Hour 00–23
%M	48	Minute 00–59
%S	55	Second 00–60
%A	Monday	Full weekday name
%B	June	Full month name
%I	04	12-hour clock hour
%p	PM	AM or PM

Code Explanation

time(NULL) — passes NULL so the function returns the current time without storing it into a pointer. The return value (a time_t) is assigned to now. You can also write time(&now) which both sets now and returns its value.
(long)now for printf — time_t might be int or long depending on the platform. Casting to long and using %ld is safe on both 32-bit and 64-bit systems.
ctime() includes the newline — ctime() and asctime() both append ‘\n’ to their output string. If you use printf("%s", ctime(&now)), do not add another ‘\n’ in the format string or you will get a blank line.
tm_year + 1900 — the struct tm year field counts years since 1900. For 2026: tm_year = 126, and 126 + 1900 = 2026. This is one of the most common off-by-1900 bugs in C time handling.
tm_mon + 1 — months are 0-based (0=January). Add 1 for human display. tm_mday is 1-based (no offset needed).
strftime(buf, sizeof(buf), fmt, local) — the sizeof(buf) argument limits the output length to prevent buffer overflow. strftime returns the number of characters written (excluding null terminator), or 0 if the buffer is too small.

What This Program Teaches

time_t is an integer — the Unix epoch representation (seconds since 1970-01-01) is just a number. You can subtract two time_t values to get the elapsed seconds between two moments.
struct tm broken-down time — use localtime() when you need to access or manipulate individual date/time components (display day of week, check if after 5pm, etc.). Use ctime() or strftime() when you just need a formatted string.
strftime for custom formats — strftime gives you full control over how dates are displayed. The ISO 8601 format (2026-06-29) is internationally unambiguous — use it when building log files, file names, or databases.
localtime vs gmtime — localtime() converts to the local timezone (set by the TZ environment variable or system setting). gmtime() converts to UTC. For timestamps in log files shared across timezones, use gmtime.

Related Programs

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C Program to demonstrate time functions.