How to get UTC time

Question

I'm programming a small little program to download the appropriate set of files to be used by a meteorological software package. The files are in format like YYYYMMDD and YYYYMMDD HHMM in UTC. I want to know the current time in UTC in C++ and I'm on Ubuntu. Is there a simple way of doing this?

Can you restate the question: do you want to _reformat_ the filenames from UTC to local? Or do you want to _access_ current time as UTC? — Dirk Eddelbuettel, Dec 16 '13 at 19:33
Just access the current time in UTC, sorry for the confusion, I'll edit the question. — Jason Mills, Dec 16 '13 at 19:34

Dirk Eddelbuettel · Accepted Answer · 2013-12-16T21:19:59.810

11

A high-end answer in C++ is to use Boost Date_Time.

But that may be overkill. The C library has what you need in strftime, the manual page has an example.

/* from man 3 strftime */

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

int main(int argc, char *argv[]) { 
    char outstr[200];
    time_t t;
    struct tm *tmp;
    const char* fmt = "%a, %d %b %y %T %z";

    t = time(NULL);
    tmp = gmtime(&t);
    if (tmp == NULL) {
        perror("gmtime error");
        exit(EXIT_FAILURE);
    }

    if (strftime(outstr, sizeof(outstr), fmt, tmp) == 0) { 
        fprintf(stderr, "strftime returned 0");
        exit(EXIT_FAILURE); 
    } 
    printf("%s\n", outstr);
    exit(EXIT_SUCCESS); 
}

I added a full example based on what is in the manual page:

$ gcc -o strftime strftime.c 
$ ./strftime
Mon, 16 Dec 13 19:54:28 +0000
$

edited Dec 16 '13 at 21:19

answered Dec 16 '13 at 19:36

Dirk Eddelbuettel

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The strftime manpage example uses localtime() though. OP will need gmtime() in its stead to get the UTC time. – nos Dec 16 '13 at 19:38
Using strftime appears to be the best looking option. Thanks. I can convert to UTC manually if necessary, but I want to look into what nos said. – Jason Mills Dec 16 '13 at 19:40
@nos How would I account for DST using gmtime()? – Jason Mills Dec 16 '13 at 19:44
@JasonMills There's no DST with UTC. time(NULL); should return the correct thing which you can pass to gmtime(); – nos Dec 16 '13 at 19:50
1

@JasonMills: (a) `gmtime` gives you UTC, not GMT, and UTC is agnostic of any time zones; (b) GMT is a single timezone anyway. :) The UK switches into BST when DST is applied. – Lightness Races in Orbit Dec 16 '13 at 19:51
I added a full example. – Dirk Eddelbuettel Dec 16 '13 at 19:56
@LightnessRacesinOrbit Thanks for reminding me, I don't know why I didn't realise that immediately, – Jason Mills Dec 16 '13 at 19:59

score 9 · Answer 2 · edited Oct 30 '20 at 21:56

9

You can use gmtime:

struct tm * gmtime (const time_t * timer);
Convert time_t to tm as UTC time

Here's an example:

std::string now()
{
  std::time_t now= std::time(0);
  std::tm* now_tm= std::gmtime(&now);
  char buf[42];
  std::strftime(buf, 42, "%Y%m%d %X", now_tm);
  return buf;
}

Output:

20131220 19:33:51

ideone link: http://ideone.com/pCKG9K

edited Oct 30 '20 at 21:56

Andrew

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answered Dec 16 '13 at 19:43

nurettin

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6

You should use `std::strftime(buf, sizeof buf, ...)`. It's also worth noting that the `return buf;` is valid only because the value is implicitly converted to a `std::string`; if `now` returned a `char*` (as it likely would in C), you'd be returning a pointer to a local object, which is a big no-no. – Keith Thompson Dec 16 '13 at 19:48
3

Note: if possible you might want to use `gmtime_r` instead, which whilst non-standard, is both re-entrant and data-race safe. – Matthieu M. Dec 16 '13 at 19:51
@MatthieuM. you're correct, I would normally have a check for the existence of gmtime_r. I thought it was overkill for this answer. Thanks for useful feedback. – nurettin Dec 16 '13 at 19:52

How to get UTC time

2 Answers2