A time zone specified in an environment variable \fBTZ\fR.
.IP [5]
The local time zone from the Control Panel on Windows systems.
.IP [6]
The C library's idea of the local time zone, as defined by the
\fBmktime\fR and \fBlocaltime\fR functions.
.PP
Whatever the source of the time zone string, the same set of rules
is used to parse it.  First, if it was obtained from a \fB%z\fR
or \fB%Z\fR format group, it is checked to see if it is one of
the strings,
.CS
 gmt     ut      utc     bst     wet     wat     at
 nft     nst     ndt     ast     adt     est     edt
 cst     cdt     mst     mdt     pst     pdt     yst
 ydt     hst     hdt     cat     ahst    nt      idlw
 cet     cest    met     mewt    mest    swt     sst
 eet     eest    bt      it      zp4     zp5     ist
 zp6     wast    wadt    jt      cct     jst     cast
 cadt    east    eadt    gst     nzt     nzst    nzdt
 idle
.CE
If it is a string in the above list, it designates a known
time zone, and is interpreted as such.
.PP

The next check is for a string beginning with a colon.

If the time zone begins with a colon, it is one of a
standardized list of names like \fB:America/New_York\fR
that give the rules for various locales.  A complete list
of the location names is too lengthy to be listed here.
On most Tcl installations, the definitions of the locations
are to be found in named files in the directory
 "\fI/no_backup/tools/lib/tcl8.5/clock/tzdata\fR".  On some Unix systems, these
files are omitted, and the definitions are instead
obtained from system files in "\fI/usr/share/zoneinfo\fR",
 "\fI/usr/share/lib/zoneinfo\fR" or "\fI/usr/local/etc/zoneinfo\fR".
As a special case, the name \fB:localtime\fR refers to
the local time zone as defined by the C library.
.PP
A string consisting of a plus or minus sign followed by
four or six decimal digits is interpreted as an offset in
hours, minutes, and seconds (if six digits are present) from
UTC.  The plus sign denotes a sign east of Greenwich;
the minus sign one west of Greenwich.
.PP
A string conforming to the Posix specification of the \fBTZ\fR
environment variable will be recognized.  The specification
may be found at
\fIhttp://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap08.html\fR.
.PP
Any other string is processed by prefixing a colon and attempting
to use it as a location name, as above.
.SH "LOCALIZATION"
Developers wishing to localize the date and time formatting and parsing
are referred to \fIhttp://tip.tcl.tk/173\fR for a
specification.
.SH "FREE FORM SCAN"
If the \fBclock scan\fR command is invoked without a \fB-format\fR