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.
"Setting the time zone" in the Configuring Your Environment chapter of the Neutrino User's Guide