topic Re: time() function dependent on timezone? in Operating System - Linux

time() function dependent on timezone?

Mohanasundaram_1 — Thu, 24 Apr 2008 05:26:17 GMT

Hi Folks,

I got this query from my programming team. This is a rp3440 system with HP-UX 11.11.

Subject: Unix System clock

Description: In HP-UX the "man 3 time" function call gives following explanation.
Time() returns the time since 00:00:00 GMT, Jan. 1, 1970, measured in seconds. This is the value of the UNIX system clock.

This means number of seconds elapsed since 1/1/1970 00:00:00 irrespective of the timezone /daylight saving.

When I execute the following program HP-UX machine :
main()
{
time_t t;
struct tm ptm;

time(&t)
printf("Timetick %d\n", t);

gmtime_r(&t, &ptm);
printf( "\n 0 gtime %04d-%02d-%02d %02d:%02d:%02d pre-isdst=0\n", ptm.tm_year + 1900,
ptm.tm_mon + 1, ptm.tm_mday, ptm.tm_hour, ptm.tm_min, ptm.tm_sec, ptm.tm_isdst );

localtime_r(&t, &ptm);
printf( "\n 0 local %04d-%02d-%02d %02d:%02d:%02d pre-isdst=0\n", ptm.tm_year + 1900,
ptm.tm_mon + 1, ptm.tm_mday, ptm.tm_hour, ptm.tm_min, ptm.tm_sec, ptm.tm_isdst );
return;
}

Output is when TZ=GMT+4
timetick 1208790892
0 gtime 2008-04-21 15:14:52 pre-isdst=0
0 local 2008-04-21 11:14:52 pre-isdst=0

Output is when TZ=TEST3:30
timetick 1208789056
0 gtime 2008-04-21 14:44:16 pre-isdst=0
0 local 2008-04-21 11:14:16 pre-isdst=0
Information Required:
from the above example time() function does depend on the timezone. Is it TRUE ? Is there any other function which will return number of seconds elapsed since 1970 at GMT+0/UTC. Which should not use the timezone or the daylight configurations.

Useful answers will be rewarded with points.

With regards,
Mohan.

Re: time() function dependent on timezone?

Steven E. Protter — Thu, 24 Apr 2008 07:51:17 GMT

Shalom Mohan,

The time() function does not have any relation to timezone setting.

Time is calculated by the number of seconds since January 1, 1970. Time zone merely controls display.

SEP

Re: time() function dependent on timezone?

Steven Schweda — Thu, 24 Apr 2008 08:18:19 GMT

If you want to know what time() does, print
the output from time() (in this case, "t"),
not the output from gmtime_r() or
localtime_r().

Re: time() function dependent on timezone?

Mohanasundaram_1 — Thu, 24 Apr 2008 08:21:37 GMT

Hi Steven,

Thanks for your reply. In such a case is it not possible to see the time irrespective of the timezone?

From the question I got, I understand that the programmer wants to display the time without the Timezone affecting its display. In other words, how can we display time irrespective of timezone, if possible.

This topic is a bit alien to me and am not sure if I am asking the correct question here.

With regards,
Mohan.

Re: time() function dependent on timezone?

Steven Schweda — Thu, 24 Apr 2008 08:53:19 GMT

gmtime[_r]() should be independent of any
time zone settings. I'd expect
localtime[_r]() to depend on the time zone.

man localtime

localtime() Correct for the time zone and any summer time zone
adjustments (such as Daylight Savings Time in the
USA), according to the contents of the TZ
environment variable (see Environment Variables
below). [...]

Re: time() function dependent on timezone?

Mohanasundaram_1 — Thu, 24 Apr 2008 09:01:55 GMT

Hi Steven Schweda,

It is a bit of co-incidence that two Stevens replied to me :-)

I have given your first reply to the people who raised the query. I will post an update based on their response.

I will assign points to you during this update. Thanks for your time.

With regards,
Mohan.

Re: time() function dependent on timezone?

Steven Schweda — Thu, 24 Apr 2008 09:13:22 GMT

After some editing of your source, I don't
see a problem with the time() or gmtime_r()
results:

td176> echo $TZ ; ./tt ; export TZ=CST6CDT ; echo $TZ ; ./tt
EST5EDT
Timetick 1209031863
gmtime 2008-04-24 10:11:03 pre-isdst=0
local 2008-04-24 06:11:03 pre-isdst=1
CST6CDT
Timetick 1209031863
gmtime 2008-04-24 10:11:03 pre-isdst=0
local 2008-04-24 05:11:03 pre-isdst=1

Re: time() function dependent on timezone?

Mohanasundaram_1 — Thu, 24 Apr 2008 09:50:06 GMT

Hi Steven Schweda,

Your Last post is encouraging. May I ask you what kind of edit was done on the source? This may solve my problem altogether.

With regards,
Mohan.

Re: time() function dependent on timezone?

Dennis Handly — Thu, 24 Apr 2008 10:21:13 GMT

>In HP-UX the "man 3 time"

That's time(2), it is a system call. ctime(3) are a libc functions.

Some minor performance tweaks: time(&t)
Don't do this, it makes the kernel sweat. Instead do:
t = time(NULL);

>printf("Timetick %d\n", t);

The type of time_t is long, so use:
printf("Timetick %ld\n", t);

>how can we display time irrespective of timezone, if possible.

You can't. All times must be relative to some time zone, possibly UTC, unless you want to see the raw time_t decimal number.

Re: time() function dependent on timezone?

Steven Schweda — Thu, 24 Apr 2008 13:38:06 GMT

> [...] what kind of edit [...]?

Enough to get it through the compiler.

td192> cat tt.c
#include
#include

main()
{
time_t t;
struct tm ptm;

time(&t);
printf("Timetick %d\n", t);

gmtime_r(&t, &ptm);
printf( "gmtime %04d-%02d-%02d %02d:%02d:%02d pre-isdst=%d\n",
ptm.tm_year + 1900, ptm.tm_mon + 1, ptm.tm_mday,
ptm.tm_hour, ptm.tm_min, ptm.tm_sec, ptm.tm_isdst );

localtime_r(&t, &ptm);
printf( "local %04d-%02d-%02d %02d:%02d:%02d pre-isdst=%d\n",
ptm.tm_year + 1900, ptm.tm_mon + 1, ptm.tm_mday,
ptm.tm_hour, ptm.tm_min, ptm.tm_sec, ptm.tm_isdst );

return 0;
}

Re: time() function dependent on timezone?

Steven Schweda — Thu, 24 Apr 2008 13:47:03 GMT

> The type of time_t is long, so use:
> printf("Timetick %ld\n", t);

And what do you need to do to make "long"
different from "int"?

Re: time() function dependent on timezone?

Dennis Handly — Fri, 25 Apr 2008 00:16:51 GMT

> Steven: And what do you need to do to make "long" different from "int"?

(Use C++? :-)
Compile in 64 bit mode (+DD64) because you want the time to go past 2038 for your mortgages.

warning #2181-D: argument is incompatible with
corresponding format string conversion
printf("Timetick %d\n", t);

Re: time() function dependent on timezone?

Mohanasundaram_1 — Sun, 04 May 2008 05:34:53 GMT

Hi Folks,

I am closing this thread as I have not heard from the originator as of now. I have given this link for them to follow and I am sure your suggestions were very useful.

Thanks for both the Steven and Dennis. The answers you have provided were informative and very useful. I have assigned points to all the people who replied.

With regards,
Mohan.