Multiple Timezones management

[purJus]

I would like to use the time library to manage multiple times with different timezones in my embedded system.

I have already seen the ble_app_cts_c example and the nrf5-calendar-example example without find the management of the timezones.

With IAR it's easy to use the RTC counter with the time library. And it's easy to manage the rules of timezone (https://www.iar.com/support/resources/articles/using-c-standard-library-time-and-clock-functions/).

But with Keil MDK-ARM or GCC ARM I don't find the solution to manage the different timezones or switch the current rule of the timezone. I don't find any API. I don't if it's necessary to write weak functions...

Can you help me?

In advance, thank you.

[haakonsh]

Use UTC/GMT for the calendar, then add or retract any number of hours given by what time zone you want to use. We have not implemented the timezone features.

[purJus]

OK, but do you known if the MDK-ARM toolchain or GCC ARM have a specific time library same like IAR?

[haakonsh]

Not that I'm aware of. I've seen the time.h file in MDK-ARM and GCC ARM, but not an actual implementation. The IAR implementation seems to use the systick feature of ARMs and that should be fairly universal, but It uses the CPU and not a LF timer like our RTC. 

[purJus]

IAR offers to use Systick or RTC counter or any counter same like MDK-ARM inside the time function declared as a weak function:

For example:

time_t time(time_t* t)
{
    time_t tmp = Systick value or RTC or...;
    if (t) *t = tmp;
    return tmp;
}

But for the timezone...?

[haakonsh]

Well you should be able to use the standard c library for the timezone functions, but I think this is already implemented in MDK-ARM. I'm guessing that all the functions are implemented as weak functions and that you can override them in order to use target specific implementation. 

-Edit:
There are full specs for the implementation of the functions in the C99 spec, chapter 7.23. http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf. 

-Edit2:
A slightly more readable example is the GNU C docs:
ftp://ftp.gnu.org/old-gnu/Manuals/glibc-2.2.3/html_chapter/libc_21.html