Get a time_t from universal time string? - c++

Question

If I have this string:
2011-10-08T07:07:09Z
is it possible to get a time_t from it? If so, how can this be done?

Answer 1

Yes, it is. First, convert it to a broken down time with strptime(3). This gives you a struct tm, which is the structure type for a broken down time.
From there, you can convert to a time_t with mktime(3).
Here's an example:
#define _XOPEN_SOURCE
#include <time.h>
#include <stdio.h>
#include <string.h>
int main(void) {
const char *date_example = "2011-10-08T07:07:09Z";
struct tm broken_down;
memset(&broken_down, 0, sizeof(broken_down));
strptime(date_example, "%Y-%m-%dT%H:%M:%SZ", &broken_down);
broken_down.tm_isdst = 0; // Indicates that DST is not in effect
time_t epoch_time = mktime(&broken_down);
// Note: this is platform dependent
printf("Epoch time: %lld\n", (long long) epoch_time);
return 0;
}

Answer 2

Use sscanf() to tear apart the time. The trick is somehow determine the difference between local and universal time so code may call mktime() - which uses assumes struct tm is local time..
#include <time.h>
#include <stdio.h>
int Get_TZ_delta(const struct tm *tmptr) {
// Make local copy
struct tm tm = *tmptr;
time_t t = mktime(&tm);
struct tm utc_tm = *gmtime(&t);
time_t t2 = mktime(&utc_tm);
return (int) difftime(t, t2);
}
time_t UniversalTimeStamp_to_time_t(const char *ts) {
struct tm tm = { 0 };
// Use a sentinel to catch extra garbage
char sentinel;
if (sscanf(ts, "%d-%2d-%2dT%2d:%2d:%2dZ%c", &tm.tm_year, &tm.tm_mon,
&tm.tm_mday, &tm.tm_hour, &tm.tm_min, &tm.tm_sec, &sentinel) != 6) {
return -1;
}
// struct tm uses offset from 1900 and January is month 0
tm.tm_year -= 1900;
tm.tm_mon--;
// Convert tm from UCT to local standard time
tm.tm_isdst = 0;
tm.tm_sec += Get_TZ_delta(&tm);
time_t t = mktime(&tm); // mktime() assumes tm is local
// test code
{
printf("UTC `%s`\n", ts);
char buf[100];
strftime(buf, sizeof buf, "%Y-%m-%dT%H:%M:%S %Z", &tm);
printf("Local %s\n", buf);
printf("Unix %lld\n\n", (long long) mktime(&tm));
}
return t;
}
int main(void) {
UniversalTimeStamp_to_time_t("2015-06-18T22:07:52Z");
UniversalTimeStamp_to_time_t("2011-10-08T07:07:09Z");
UniversalTimeStamp_to_time_t("1970-01-01T00:00:00Z");
return 0;
}
Output
UTC `2015-06-18T22:07:52Z`
Local 2015-06-18T17:07:52 CDT
Unix 1434665272
UTC `2011-10-08T07:07:09Z`
Local 2011-10-08T02:07:09 CDT
Unix 1318057629
UTC `1970-01-01T00:00:00Z`
Local 1969-12-31T18:00:00 CST
Unix 0

Answer 3

Another approach works should code know that time_t is the number of seconds since Jan 1, 1970 0:00:00. Uses sscanf() to parse the string, calculate the number of days, and then return the number of seconds.
#include <time.h>
#include <stdio.h>
#define MARCH 3
#define DaysPer400Years (400*365LL + 97)
#define DaysPer100Years (100*365LL + 24)
#define DaysPer4Years (4*365LL + 1)
#define DaysPer1Year 365LL
#define DayNumber1970Jan1 719469LL
long long DayNumber(int year, int Month, int Day, long epoch) {
long long dn = Day;
long long y = year;
y += Month / 12;
Month %= 12;
while (Month < MARCH) {
Month += 12;
y--;
}
// And then a miracle occurs.
dn += ((Month - MARCH) * (7832 / 4) + (140 / 4)) >> (8 - 2);
dn += (y / 400) * DaysPer400Years;
y %= 400;
dn += (y / 100) * DaysPer100Years;
y %= 100;
dn += (y / 4) * DaysPer4Years;
y %= 4;
dn += y * DaysPer1Year;
return dn - epoch;
}
time_t UniversalTimeStamp_to_time_t(const char *ts) {
int y,m,d,H,M,S;
// Use a sentinel to catch extra garbage
char sentinel;
if (sscanf(ts, "%d-%2d-%2dT%2d:%2d:%2dZ%c", &y, &m,
&d, &H, &M, &S, &sentinel) != 6) {
return -1;
}
long long t = DayNumber(y, m, d, DayNumber1970Jan1);
t = t*24L*60*60 + 3600L*H + 60*M + S;
// test code
{
printf("UTC `%s`\n", ts);
time_t tt = t;
struct tm tm = *gmtime(&tt);
char buf[100];
strftime(buf, sizeof buf, "%Y-%m-%dT%H:%M:%S %Z", &tm);
printf("Local %s\n", buf);
printf("Unix %lld\n\n", t);
}
return t;
}
int main(void) {
UniversalTimeStamp_to_time_t("2015-06-18T22:07:52Z");
UniversalTimeStamp_to_time_t("2011-10-08T07:07:09Z");
UniversalTimeStamp_to_time_t("1970-01-01T00:00:00Z");
return 0;
}
Output
UTC `2015-06-18T22:07:52Z`
Local 2015-06-18T22:07:52
Unix 1434665272
UTC `2011-10-08T07:07:09Z`
Local 2011-10-08T07:07:09
Unix 1318057629
UTC `1970-01-01T00:00:00Z`
Local 1970-01-01T00:00:00
Unix 0