tzname0

tzname0 is a commonly used shorthand referring to the first element of the tzname array in many C and POSIX environments. The tzname array typically consists of two strings: tzname[0] contains the abbreviation for standard time, and tzname[1] contains the abbreviation for daylight saving time (if DST is observed). In practice, tzname0 is used to denote tzname[0], the standard-time name currently in effect for the active time zone.

The values of tzname and tzname0 are determined by the system’s time zone data and are initialized when a program starts or when the tzset function is invoked. This initialization may rely on the TZ environment variable or on the system’s time zone database. As a result, the actual strings (for example, "EST", "CET", "GMT" or "UTC") depend on the locale, the time zone, and whether DST rules apply.

Platform differences exist. In POSIX and most Unix-like systems, extern char *tzname[2] is declared in time.h. Windows uses a separate symbol, _tzname[2], as part of its C runtime. Because tzname is not part of the ISO C standard, its presence and exact behavior are implementation-defined and can vary between platforms and libraries.

Usage considerations. Relying on tzname0 for portable code is discouraged; it is better to use standard, portable facilities such as strftime with the %Z format specifier to obtain the time zone abbreviation, or to query TZ and DST rules through well-supported APIs. Reading tzname[0] directly may also raise concerns about thread safety and consistency across library versions.

See also TZ environment variable, tzset, strftime, time.h, _tzname.