_daylight, _timezone, and _tzname are used in some time and date routines to make local-time adjustments. They are declared in TIME.H as
extern int _daylight;
extern long _timezone;
extern char *_tzname[2];
On a call to _ftime, localtime, or _tzset, the values of _daylight, _timezone, and _tzname are determined from the value of the TZ environment variable. If you do not explicitly set the value of TZ, _tzname[0] and _tzname[1] contain empty strings, but the time-manipulation functions (_tzset, _ftime, and localtime) attempt to set the values of _daylight and _timezone by querying the operating system for the default value of each variable. The time-zone global variable values are as follows.
| Variable | Value |
| _daylight | Nonzero if daylight-saving-time zone (DST) is specified in TZ; otherwise, 0. Default value is 1. |
| _timezone | Difference in seconds between coordinated universal time and local time. Default value is 28,800. |
| _tzname[0] | Time-zone name derived from TZ environment variable. |
| _tzname[1] | DST zone name derived from TZ environment variable. Default value is PDT (Pacific daylight time). If DST zone is omitted from TZ, _tzname[1] is empty string. |