time.h

Learn date and time with time.h.

time.h

The <time.h> header provides date and time operations: time_t (time type), struct tm (broken-down time) and mktime (converts broken-down time to a timestamp).

struct tm splits a date into parts: tm_year (years since 1900), tm_mon (month 0–11), tm_mday (day of month), tm_wday (day of week 0–6; 0 is Sunday). mktime(&t) computes and fills missing fields (such as tm_wday). Example: struct tm t = {0}; t.tm_year = 100; t.tm_mon = 0; t.tm_mday = 1; mktime(&t);. After this t.tm_wday gives the weekday of January 1, 2000. time(NULL) returns the current timestamp; but this value changes each run, so it's not deterministic.

Finding weekday with mktime

mktime not only gives a timestamp; it also fills in missing fields in struct tm. This lets us find the weekday of a given date.

For January 1, 2000: struct tm t = {0}; t.tm_year = 100; t.tm_mon = 0; t.tm_mday = 1; mktime(&t);. After the call t.tm_wday is 6; because January 1, 2000 was a Saturday (0=Sunday, 6=Saturday). Initializing with struct tm t = {0} is important; otherwise unused fields may hold garbage and mktime may give wrong results. This technique is common for finding the weekday of a specific date; used in calendar or scheduling apps.

Exercise Find the weekday

In main start with struct tm t = {0};. Set t.tm_year = 100;, t.tm_mon = 0;, t.tm_mday = 1;. Call mktime(&t);. Print with printf("Day: %d\n", t.tm_wday);.

Show example solution

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

int main(void) {
    struct tm t = {0};
    t.tm_year = 100;
    t.tm_mon = 0;
    t.tm_mday = 1;
    mktime(&t);
    printf("Day: %d\n", t.tm_wday);
    return 0;
}

Test yourself

In struct tm, what month does tm_mon = 0 denote?
- February
- January
- New Year
- December
Show answer

January — tm_mon ranges 0–11; 0 is January.

Year setting

t.tm_____ = 100;  // year 2000

Show answer

t.tm_year = 100;  // year 2000

struct tm fields

tm_year

year (since 1900)
tm_mon

month (0–11)
tm_mday

day of month
tm_wday

day of week

Next lesson: ctype.h

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