将UTC时间转换为C中的Unix时间戳

Question

I need to know the current UTC time and convert it to Unix time stamp. I tried using combination of time() , mktime() , gmtime() but not getting correct UTC time Following is the snippet I am using:

time_t now;
struct tm  ts;
char buf[80];
now = time(NULL);
ts = *gmtime(&now);
strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S %Z", &ts);

This gives me following output as --> Time :: Sat 2015-01-10 00:14:34 CST which is adding 5 Hr 30 minutes into local time instead of subtracting 5 Hr 30 min. Can anyone tell me why this is happening ?

Answer 1

[edit] Re-thought the issue.

OP wants a Unix time stamp . Such a time stamp is the seconds since Jan 1, 1970. This is likely nothing more than the below.

sprintf(buffer, "%lld", (long long) time(NULL));

OTOH, OP also seems to want year-month-day hour:minute:second. This is not a UNIX time stamp. It is simply a time stamp which the rest of this answer discusses a portable way to derive.

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time() returns some real type capable of representing time. It is very often some integer type representing the numbers of seconds since Jan 1, 1970 0:00:00 UTC (or GMT ).

 now = time(NULL);

gmtime() coverts time_t into a year, month, day, etc. structure for the UTC (GMT) timezone.

ts = *gmtime(&now);

---

Now comes a coding problem:

strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S %Z", &ts);

strftime() may ( @Jasen ) not known the timezone from &ts . Instead, it gets the locale's timezone. That result is adjusted per the is_daylight_time field ts.tm_isdst . So the following does not make sense.

ts = *gmtime(&now);  // UTC/GMT time
strftime(buf, sizeof(buf), "... %Z ...", &ts);  // print local standard time name

Instead use

ts = *gmtime(&now);
strftime(buf, sizeof(buf), "%a %Y-%m-%d %H:%M:%S UTC", &ts);

---

Consider ISO 8601 formats

strftime(buf, sizeof(buf), "%Y-%m-%dT%H:%M:%SZ", &ts);
//  2015-01-06T03:51:57Z

Answer 2

The return value of time() already is in UTC:

time() returns the time as the number of seconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

It's not clear to me why you're doing further processing.

Answer 3

Fix Your Clock

Your time zone is IST "Asia/Calcutta" (I couldn't find anything else with a 5:30 offset from GMT)

But it's printing as GMT as CST ths suggests that your computers clock is badly misconfigured.

The code you posted works fine here. I cannot provoke it to print CST instead of GMT.