Relationship between TZ or _TZ and LC_TOD

The C/C++ runtime library uses time zone information specified by the TZ or _TZ environment variable to convert universal reference times to local times. When neither the TZ nor _TZ variable is defined, the C/C++ runtime library uses time zone information specified by the LC_TOD category of the current locale to map universal reference times to local times. If LC_TOD in the current locale has not been customized, the C/C++ runtime library uses Coordinated Universal Time (UTC) for POSIX programs or, for non-POSIX programs, the time zone of the system on which C/C++ is installed.

Note: The time zone external variables, tzname, timezone, and daylight, declarations remain feature test protected in time.h. Definition of these external variables are only known to the C/C++ runtime library if the z/OS® UNIX System Services C/C++ signature CSECT is link edited with your C/C++ application.