Converting local time to UTC

Mark

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Oct 14, 2015, 2:52:06 AM10/14/15

to

I am trying to write some code to convert local time to UTC. I am
using the following which works in many circumstance but a client in
the US has found it not working correctly:

void convertToUTC(const char *input, char *output)
{
struct tm tm_;

memset(&tm_, 0, sizeof(struct tm));

if (!strptime(input, "%d-%b-%Y %T", &tm_))
{
// failed
return;
}
time_t thetime = mktime(&tm_);
// Convert to UTC
struct tm gmcal = *gmtime(&thetime);
thetime = mktime(&gmcal);

struct tm *tm_p;
struct tm tm2;

tm_p = gmtime_r(&thetime, &tm2);

sprintf(output, "%4d-%02d-%02dT%02d:%02d:%02dZ",
tm_p->tm_year + 1900,
tm_p->tm_mon + 1,
tm_p->tm_mday,
tm_p->tm_hour,
tm_p->tm_min,
tm_p->tm_sec);
}

"13-Oct-2015 06:57:17" --> "2015-10-13T22:57:17Z"

Output from 'date' command:
date; date -u
Tue Oct 13 06:58:10 PDT 2015
Tue Oct 13 13:58:10 UTC 2015

Can someone tell me how to fix this?

Lőrinczy Zsigmond

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Oct 14, 2015, 5:30:12 AM10/14/15

to

I think it could be simplified

void convertToUTC(const char *input, char *output)
{

struct tm tmloc, tmutc;
time_t thetime;

memset(&tmloc, 0, sizeof tmloc);
memset(&tmutc, 0, sizeof tmutc);

if (!strptime(input, "%d-%b-%Y %T",&tmloc))
{
// failed
return;
}
thetime = mktime(&tmloc);
gmtime_r(&thetime, &tmutc);

sprintf(output, "%4d-%02d-%02dT%02d:%02d:%02dZ",

tmutc.tm_year + 1900,
tmutc.tm_mon + 1,
tmutc.tm_mday,
tmutc.tm_hour,
tmutc.tm_min,
tmutc.tm_sec);
}

Mark

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Oct 14, 2015, 5:38:25 AM10/14/15

to

Won't this behave in exactly the same way as my code?

Nobody

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Oct 14, 2015, 10:01:48 AM10/14/15

to

On Wed, 14 Oct 2015 07:51:40 +0100, Mark wrote:

> I am trying to write some code to convert local time to UTC. I am
> using the following which works in many circumstance but a client in
> the US has found it not working correctly:
>
> void convertToUTC(const char *input, char *output)
> {
> struct tm tm_;
>
> memset(&tm_, 0, sizeof(struct tm));
>
> if (!strptime(input, "%d-%b-%Y %T", &tm_))
> {
> // failed
> return;
> }
> time_t thetime = mktime(&tm_);

You should probably set tm_.is_dst=-1 prior to calling mktime(). That will
cause it to attempt to determine whether or not DST is in effect based
upon the date and time. That still has a problem when clocks go back as
any time within the last hour of DST or the first hour afterwards will be
ambiguous.

If the is_dst is field zero, DST is not used.

But that's not the biggest issue here.

> // Convert to UTC
> struct tm gmcal = *gmtime(&thetime);
> thetime = mktime(&gmcal);
>
> struct tm *tm_p;
> struct tm tm2;
>
> tm_p = gmtime_r(&thetime, &tm2);

This is bogus. You're taking a time_t (number of seconds since the epoch),
converting it to calendar time as UTC, then requesting the calendar time
be treated as local time and converted to a time_t. Then converting it to
calendar time as UTC again.

The second mktime() call and the gmtime_r() call shouldn't be there. If
you want local time, it's in tm_. If you want UTC, it's in gmcal. The
value in tm2/*tm_p is nonsense; it's had the timezone offset applied twice.

I can't see how this can work in any situation other than where local time
happens to be UTC (or possibly UTC+1 due to DST which is being ignored).

Richard

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Oct 16, 2015, 2:53:22 PM10/16/15

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[Please do not mail me a copy of your followup]

Mark <i...@dontgetlotsofspamanymore.net> spake the secret code
<bdur1bh61v892is7r...@4ax.com> thusly:

>I am trying to write some code to convert local time to UTC. I am
>using the following which works in many circumstance but a client in
>the US has found it not working correctly:
>
>void convertToUTC(const char *input, char *output)

C-style string char * arguments and C library functions?

Are you sure you're posting to the correct newsgroup?

Did you mean comp.lang.c instead?

This sort of stuff is *trivial* with boost.datetime. They even have
an explicit example demonstrating this exact conversion.

<http://www.boost.org/doc/libs/1_59_0/doc/html/date_time/examples.html#date_time.examples.local_utc_conversion>

No need to use crufty, error-prone, non-typesafe hacks with sprintf.

For well known problems, always prefer a time-tested well known
implementation.
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