CODEWITHSUNDEEP
ctime
Timothy Harmon
Timothy Harmon

Reputation: 21

Date/Time into time_t in C

I am trying to calculate the time, in seconds, for a certain timestamp. How can I input a date and time into time_t so I can calculate the time from time(null)? (time(null) is 01/01/1970 if I understand it right)

Upvotes: 1

Views: 2004

Answers (2)

dlchambers
dlchambers

Reputation: 3752

Adafruit's RTCLib has a unixtime function:

RTC_DS3231 _rtc; // or whichever chip you're using
_rtc.begin();
uint32_t unixtm = _rtc.now().unixtime();

Upvotes: 0

Steve Summit
Steve Summit

Reputation: 48083

Here is a simple example that computes the time more or less now, where I am:

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

int main()
{
    time_t t;
    struct tm tm;

    /* fill in values for 2019-08-22 23:22:26 */
    tm.tm_year = 2019 - 1900;
    tm.tm_mon = 8 - 1;
    tm.tm_mday = 22;
    tm.tm_hour = 23;
    tm.tm_min = 22;
    tm.tm_sec = 26;
    tm.tm_isdst = -1;
    t = mktime(&tm);
    printf("%ld\n", t);
}

As you can see, the values in some of the fields use mildly strange conventions: tm_mon is 0-based, and tm_year counts from 1900. Setting tm_isdst to -1 means "I'm not sure if DST applied on August 22; you figure it out".

When I run it, this program prints 1566530546, which is indeed the number of seconds since midnight UTC on January 1, 1970. (If you run the program, though, you'll likely get a slightly different number, because it will work from 23:22:26 in your time zone.)

As the preceding paragraphs suggest, the mktime function does take your time zone into account, as well as any necessary DST correction. If you want to do a similar conversion without these corrections, there's an analogous function called timegm, although it's not Standard and not present on all systems.

The tm_wday and tm_yday (day of week and day of year) fields are ignored when you call mktime, although they will have been filled in with their correct values when the function returns.

Upvotes: 3

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