time.h
- C course
- Intermediate
- Lesson 85/116
- 5 min
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.
Practice 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);.
Starter code
#include <stdio.h>
#include <time.h>
// main: declare struct tm and zero it
// tm_year = 100 (year 2000), tm_mon = 0 (January), tm_mday = 1
// normalize with mktime, print tm_wday
int main(void) {
// write here
return 0;
}Sample 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;
}Expected output
Day: 6
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 In struct tm, what month does tm_mon = 0 denote?
- February
- January Correct answer
- New Year
- December
Explanation: tm_mon ranges 0–11; 0 is January.
2 Year setting
t.tm_____ = 100; // year 2000t.tm_year = 100; // year 2000Explanation: tm_year counts from 1900; 100 → 2000.
3 struct tm fields
| Term | Meaning |
|---|---|
tm_year |
year (since 1900) |
tm_mon |
month (0–11) |
tm_mday |
day of month |
tm_wday |
day of week |