The time zone information files used by tzset(3) begin with the magic characters "TZif" to identify then as time zone information files, followed by a character identifying the version of the file's format (as of 2005, either an ASCII NUL ('\0') or a '2') followed by fifteen bytes containing zeroes reserved for future use, followed by six four-byte values of type long, written in a "standard" byte order (the high-order byte of the value is written first). These values are, in order:
The above header is followed by tzh_timecnt four-byte values of type long, sorted in ascending order. These values are written in "standard" byte order. Each is used as a transition time (as returned by time(2)) at which the rules for computing local time change. Next come tzh_timecnt one-byte values of type unsigned char; each one tells which of the different types of "local time" types described in the file is associated with the same-indexed transition time. These values serve as indices into an array of ttinfo structures (with tzh_typecnt entries) that appear next in the file; these structures are defined as follows:
struct ttinfo {
long tt_gmtoff;
int tt_isdst;
unsigned int tt_abbrind;
};
Each structure is written as a four-byte value for tt_gmtoff of type long, in a standard byte order, followed by a one-byte value for tt_isdst and a one-byte value for tt_abbrind. In each structure, tt_gmtoff gives the number of seconds to be added to UTC, tt_isdst tells whether tm_isdst should be set by localtime(3), and tt_abbrind serves as an index into the array of timezone abbreviation characters that follow the ttinfo structure(s) in the file.
Then there are tzh_leapcnt pairs of four-byte values, written in standard byte order; the first value of each pair gives the time (as returned by time(2)) at which a leap second occurs; the second gives the total number of leap seconds to be applied after the given time. The pairs of values are sorted in ascending order by time.
Then there are tzh_ttisstdcnt standard/wall indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as standard time or wall clock time, and are used when a timezone file is used in handling POSIX-style timezone environment variables.
Finally, there are tzh_ttisgmtcnt UTC/local indicators, each stored as a one-byte value; they tell whether the transition times associated with local time types were specified as UTC or local time, and are used when a timezone file is used in handling POSIX-style timezone environment variables.
localtime(3) uses the first standard-time ttinfo structure in the file (or simply the first ttinfo structure in the absence of a standard-time structure) if either tzh_timecnt is zero or the time argument is less than the first transition time recorded in the file.
It seems that timezone uses tzfile internally, but glibc refuses to expose it to userspace. This is most likely because the standardised functions are more useful and portable, and actually documented by glibc. It may only be in glibc just to support the non-glibc-maintained timezone data (which is maintained by some other entity).
For version-2-format timezone files, the above header and data is followed by a second header and data, identical in format except that eight bytes are used for each transition time or leap-second time. After the second header and data comes a newline-enclosed, POSIX-TZ-environment-variable-style string for use in handling instants after the last transition time stored in the file (with nothing between the newlines if there is no POSIX representation for such instants).