void set_system_time

From a Clock / Calendar type RTC:

        struct tm rtc_time;

        read_rtc(&rtc_time);
        rtc_time.tm_isdst = 0;
        set_system_time( mktime(&rtc_time) );

From a Network Time Protocol time stamp:

        set_system_time(ntp_timestamp - NTP_OFFSET);

From a UNIX time stamp:

        set_system_time(unix_timestamp - UNIX_OFFSET);