21.4.8 Functions and Variables for Time Zones
The following variables are defined for compatibility with System V Unix. Like tzname, these variables are set by calling tzset or the other time conversion functions.
— Variable: long int timezone
This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is 5*60*60. Unlike the tm_gmtoff member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use tm_gmtoff, since it contains the correct offset even when it is not the latest one.