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TIMEZONE(3) Library Functions Manual TIMEZONE(3)

timezone
return the time zone abbreviation

#include <time.h>
char *
timezone(int zone, int dst);

This interface is for compatibility only; it is impossible to reliably map timezone()'s arguments to a time zone abbreviation (see ctime(3)).
The timezone() function returns a pointer to a time zone abbreviation for the specified zone and dst values. zone is the number of minutes west of GMT and dst is non-zero if Daylight Saving Time is in effect.
This function is not used to set the system's time zone. The default system time zone may be set by running “zic -l timezone” as the superuser.

ctime(3), zic(8)

A timezone() function appeared in Version 7 AT&T UNIX.
June 5, 2013 OpenBSD-5.6