Daylight saving time
The IANA time zone database maps a name to the named location's historical and predicted clock shifts. This database is used by many computer software systems, including most Unix-like operating systems, Java, and the Oracle RDBMS; HP's "tztab" database is similar but incompatible. When temporal authorities change DST rules, zoneinfo updates are installed as part of ordinary system maintenance. In Unix-like systems the TZ environment variable specifies the location name, as in TZ=':America/New_York'. In many of those systems there is also a system-wide setting that is applied if the TZ environment variable isn't set: this setting is controlled by the contents of the /etc/localtime file, which is usually a symbolic link or hard link to one of the zoneinfo files. Internal time is stored in timezone-independent epoch time; the TZ is used by each of potentially many simultaneous users and processes to independently localize time display.

Q: What environment variable defines a location's name?
TZ