1.\" $NetBSD: tzfile.5,v 1.23 2015/10/09 17:21:45 christos Exp $ 2.\" 3.\" This file is in the public domain, so clarified as of 4.\" 1996-06-05 by Arthur David Olson (arthur_david_olson@nih.gov). 5.Dd October 6, 2014 6.Dt TZFILE 5 7.Os 8.Sh NAME 9.Nm tzfile 10.Nd time zone information 11.Sh DESCRIPTION 12The time zone information files used by 13.Xr tzset 3 14begin with the magic characters 15.Dq TZif 16to identify them as time zone information files, 17followed by a character identifying the version of the file's format 18(as of 2013, either an ASCII NUL or a '2', or '3') 19followed by fifteen bytes containing zeroes reserved for future use, 20followed by six four-byte values of type 21.Fa long , 22followed by six four-byte integer values written in a standard 23byte order (the high-order byte of the value is written first). 24These values are, in order: 25.Bl -tag -width XXXXXX -compact 26.It Va tzh_ttisgmtcnt 27The number of UT/local indicators stored in the file. 28.It Va tzh_ttisstdcnt 29The number of standard/wall indicators stored in the file. 30.It Va tzh_leapcnt 31The number of leap seconds for which data entries are stored in the file. 32.It Va tzh_timecnt 33The number of transition times for which data entries are stored 34in the file. 35.It Va tzh_typecnt 36The number of local time types for which data entries are stored 37in the file (must not be zero). 38.It Va tzh_charcnt 39The number of characters of time zone abbreviation strings 40stored in the file. 41.El 42.Pp 43The above header is followed by 44.Va tzh_timecnt 45four-byte signed integer values sorted in ascending order. 46These values are written in 47These values are written in standard byte order. 48Each is used as a transition time (as returned by 49.Xr time 3 ) 50at which the rules for computing local time change. 51Next come 52.Va tzh_timecnt 53one-byte unsigned integer values; 54each one tells which of the different types of local time types 55described in the file is associated with the time period 56starting with the same-indexed transition time. 57These values serve as indices into an array of 58.Fa ttinfo 59structures (with 60.Va tzh_typecnt 61entries) that appears next in the file; 62these structures are defined as follows: 63.Bd -literal 64struct ttinfo { 65 int32_t tt_gmtoff; 66 unsigned char tt_isdst; 67 unsigned char tt_abbrind; 68}; 69.Ed 70Each structure is written as a four-byte signed integer value for 71.Va tt_gmtoff 72in a standard byte order, followed by a one-byte value for 73.Va tt_isdst 74and a one-byte value for 75.Va tt_abbrind . 76In each structure, 77.Va tt_gmtoff 78gives the number of seconds to be added to UT, 79.Va tt_isdst 80tells whether 81.Va tm_isdst 82should be set by 83.Xr localtime 3 84and 85.Va tt_abbrind 86serves as an index into the array of time zone abbreviation characters 87that follow the 88.Va ttinfo 89structure(s) in the file. 90.Pp 91Then there are 92.Va tzh_leapcnt 93pairs of four-byte values, written in standard byte order; 94the first value of each pair gives the time 95(as returned by 96.Xr time 3 ) 97at which a leap second occurs; 98the second gives the 99.Em total 100number of leap seconds to be applied during the time period 101starting at the given time. 102The pairs of values are sorted in ascending order by time. 103.Pp 104Then there are 105.Va tzh_ttisstdcnt 106standard/wall indicators, each stored as a one-byte value; 107they tell whether the transition times associated with local time types 108were specified as standard time or wall clock time, 109and are used when a time zone file is used in handling POSIX-style 110time zone environment variables. 111.Pp 112Finally there are 113.Va tzh_ttisgmtcnt 114UT/local indicators, each stored as a one-byte value; 115they tell whether the transition times associated with local time types 116were specified as UT or local time, 117and are used when a time zone file is used in handling POSIX-style 118time zone environment variables. 119.Pp 120.Xr localtime 3 121uses the first standard-time 122.Fa ttinfo 123structure in the file 124(or simply the first 125.Fa ttinfo 126structure in the absence of a standard-time structure) 127if either 128.Va tzh_timecnt 129is zero or the time argument is less than the first transition time recorded 130in the file. 131.Pp 132For version-2-format time zone files, 133the above header and data are followed by a second header and data, 134identical in format except that 135eight bytes are used for each transition time or leap second time. 136After the second header and data comes a newline-enclosed, 137POSIX-TZ-environment-variable-style string for use in handling instants 138after the last transition time stored in the file 139(with nothing between the newlines if there is no POSIX representation for 140such instants). 141.Pp 142For version-3-format time zone files, the POSIX-TZ-style string may 143use two minor extensions to the POSIX TZ format, as described in 144.Xr tzset 3 . 145First, the hours part of its transition times may be signed and range from 146\-167 through 167 instead of the POSIX-required unsigned values 147from 0 through 24. 148Second, DST is in effect all year if it starts 149January 1 at 00:00 and ends December 31 at 24:00 plus the difference 150between daylight saving and standard time. 151.Pp 152Future changes to the format may append more data. 153.Sh SEE ALSO 154.Xr ctime 3 , 155.Xr localtime 3 , 156.Xr time 3 , 157.Xr tzset 3 , 158.Xr zdump 8 159.Xr zic 8 160.\" @(#)tzfile.5 8.3 161.\" This file is in the public domain, so clarified as of 162.\" 1996-06-05 by Arthur David Olson. 163