How do you set the Time Zone offset?

[Alberto]

Does anyone know how to inform the OS the time zone offset? This is my situation. I need to produce strings with local time and UTC time in my MOD5270. I set the time with SetNTPTime() and that works fine, but how do you tell the OS that my timezone is -8:00. This sample code fragment ilustrates the situation.

Code: Select all

  time_t  tt = time(NULL);
  
  iprintf( "UTC: %s\r\n" asctime( gmtime(&tt) ) );
  iprintf( "Loc: %s\r\n" asctime( localtime(&tt) ) );

When you run this code, you get identical strings since the OS thinks UTC and local time are the same. I have not found the source code for gmtime() or localtime(), so I don't know how they are implemented internally. Any suggestions?

Regards,
Alberto

[Ridgeglider]

I posted this about a week ago to the old user group. I've pasted the same material here. Hope it helps...

GNU C has some built-in functionality here...
I occasionally modify the environment variable that sets timezones.
You can do this by the call:
putenv("TZ=EST+5EDT,M4.1.0/M10.5.0");

you can read the TZ environmental variable back by:
iprintf( "Timezone:%s\r\n", getenv("TZ"));

This allows call likes gmtime() to reflect gmt, and localtime() to
work with a time spec'd by the TZ.

A brief discussion of the TZ string in the example above:
Eastern time zone in the United States, including the appropriate
daylight saving time and its dates of applicability. The normal offset
from GMT is 5 hours; since this is west of the prime meridian, the
sign is positive. Summer time begins on the first Sunday in April at
2:00am, and ends on the last Sunday in October at 2:00am. Hence:
EST+5EDT,M4.1.0/M10.5.0

More thoroughly:
std offset dst [offset],start[/time],end[/time]
The initial std and offset specify the standard time zone, as described
above. The dst string and offset specify the name and offset for the
corresponding daylight savings time time zone; if the offset is omitted, it
defaults to one hour ahead of standard time.
The remainder of the specification describes when daylight savings
time is in effect. The start field is when daylight savings time goes into effect and the end
field is when the change is made back to standard time. The following
formats are recognized for these fields:
Jn
This specifies the Julian day, with n between 1 and 365. February 29
is never counted, even in leap years.
n
This specifies the Julian day, with n between 0 and 365. February 29
is counted in leap years.
Mm.w.d
This specifies day d of week w of month m . The day d must be between
0 (Sunday) and 6. The week w must be between 1 and 5; week 1 is the
first week in which day d occurs, and week 5 specifies the last d day in the month.
The month m should be between 1 and 12.
The time fields specify when, in the local time currently in effect,
the change to the other time occurs. If omitted, the default is 02:00:00.

The best description of this and other related facilities I've found
is at: http://www.cs.utah.edu/dept/old/texinfo ... tml#SEC309

[Alberto]

Nice answer Ridgeglider. It works like a champ. You answer was straight to the point, with examples and documentation. It doesn't get any better than that.

Alberto