How to Convert Unix Timestamp using C

Convert from epoch to human-readable date

How to convert epoch / Unix timestamps to normal readable date/time using C.

Example C routine using STRFTIME. STRFTIME converts information from a time structure to a string form, and writes the string into the memory area pointed to by “string”.

#include <stdio.h>
#include <time.h>
 
int main(void)
{
    time_t     now;
    struct tm  ts;
    char       buf[80];
 
    // Get current time
    time(&now);
 
    // Format time, "ddd yyyy-mm-dd hh:mm:ss zzz"
    ts = *localtime(&now);
    strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S %Z", &ts);
    printf("%s\n", buf);
    return 0;
}

With a custom epoch (here 1234567890):

#include <stdio.h>
#include <time.h>
 
int main(void)
{
    time_t rawtime = 1234567890;
    struct tm  ts;
    char       buf[80];
 
    // Format time, "ddd yyyy-mm-dd hh:mm:ss zzz"
    ts = *localtime(&rawtime);
    strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S %Z", &ts);
    printf("%s\n", buf);
    return 0;
}

Convert from human-readable date to epoch

#include <stdio.h>
#include <time.h>
 
int main(void) {
    struct tm t;
    time_t t_of_day;
 
    t.tm_year = 2026-1900;  // Year - 1900
    t.tm_mon = 7;           // Month, where 0 = jan
    t.tm_mday = 8;          // Day of the month
    t.tm_hour = 16;
    t.tm_min = 11;
    t.tm_sec = 42;
    t.tm_isdst = -1;        // Is DST on? 1 = yes, 0 = no, -1 = unknown
    t_of_day = mktime(&t);
 
    printf("seconds since the Epoch: %ld\n", (long) t_of_day);
}