Convert time structure to rawtime

Question

I'm trying to write a program that asks the user for time input and then converts it to rawtime.

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

int main(){

    int year, month, month_day, day, hour, minute, second;

    printf("Enter the following data:\n");

    printf("Which year?: ");
    scanf("%d", &year);

    printf("Which month?: ");
    scanf("%d", &month);

    printf("Which day?: ");
    scanf("%d", &month_day);

    printf("Which hour?: ");
    scanf("%d", &hour);

    printf("Which minute?: ");
    scanf("%d", &minute);

    printf("Which second?: ");
    scanf("%d", &second);

    struct tm givenTime;
    givenTime.tm_year = year;
    givenTime.tm_mon = month;
    givenTime.tm_mday = month_day;
    givenTime.tm_hour = hour;
    givenTime.tm_min = minute;
    givenTime.tm_sec = second;

    //Now I need to convert given input to rawtime, i.e turn all the input into seconds passed since Jan 1, 1970

mktime if you want to interpret the input as in the local time zone, gmtime if you want to interpret the input as GMT. https://en.cppreference.com/w/c/chrono — Schwern, Mar 24 '22 at 00:01
Note that `struct tm` encodes the year and month weirdly — the year 2022 is encoded as 122, and the month January is encoded as 0. — Jonathan Leffler, Mar 24 '22 at 00:30
Does the following post answers your question? [how to convert datetime to unix timestamp in c?](https://stackoverflow.com/questions/1002542/how-to-convert-datetime-to-unix-timestamp-in-c) — Paulo Pereira, Mar 24 '22 at 01:10
@Schwern [gmtime if you want to interpret the input as GMT](https://stackoverflow.com/questions/71595466/convert-time-structure-to-rawtime/71596201#comment126536785_71595466) is amiss. `gmtime()` is used to convert `time_t` to `struct tm`, not `struct tm` to `time_t`. Oddly, standard C lib lacks a `mkgmtime()`. — chux - Reinstate Monica, Mar 24 '22 at 08:15

chux - Reinstate Monica · Answer 1 · 2022-03-24T02:40:37.727

Use mktime() to convert the Y-M-D H-M-S into time_t, which is usually the integer seconds since 1970, universal time.

struct tm references months since January and years since 1900. struct tm also has a member for daylight savings time. Use 0 for standard time and 1 for daylight time. Set that to -1 if you are not certain about the daylight time status of your time stamp.

Since struct tm may have other important members, best to initialize all to 0.

#include <time.h>

struct tm givenTime = { 0 };
givenTime.tm_year = year - 1900;
givenTime.tm_mon = month - 1;
givenTime.tm_mday = month_day;
givenTime.tm_hour = hour;
givenTime.tm_min = minute;
givenTime.tm_sec = second;
givenTime.tm_isdst = -1;

time_t now = mktime(&givenTime);
if (now == -1) {
  puts("Conversion failed.");
}

// Implementation dependent output
printf("time_t now %lld\n", (long long) now);

To properly determine the seconds since some epoch like Jan 1, 1970, subtract two time_t with double difftime() which always returns the difference in seconds.

// Jan 1, 1970 0:00:00 local time
struct tm epochTime = { 0 };
epochTime.tm_year = 1970 - 1900;
epochTime.tm_mon = 1 - 1;
epochTime.tm_mday = 1;
epochTime.tm_hour = 0;
epochTime.tm_min = 0;
epochTime.tm_sec = 0;
epochTime.tm_isdst = -1;
time_t time0 = mktime(&epochTime);

double dif = difftime(now , time0);
printf("dif %.0f seconds\n", dif);

Convert time structure to rawtime

1 Answers1