Indicator of support for daylight saving time in the locale
#include <time.h> unsigned int daylight;
This global variable has a value of 1 when daylight saving time is supported in this locale, and 0 otherwise. Whenever you call a time function, tzset() is called to set the variable, based on the current time zone.
POSIX 1003.1 XSI
timezone, tzname, tzset()
“Setting the time zone” in the Configuring Your Environment chapter of the Neutrino User's Guide