Xchrono is a multi-timezone, multi-face clock program for X Windows.
Several cities have been compiled into xchrono, and can be invoked with
command-line arguments, xchrono -help gives:

Usage: xchrono [-analog] [-bw <pixels>] [-digital]
	       [-fg <color>] [-bg <color>] [-hd <color>]
	       [-hl <color>] [-bd <color>]
	       [-fn <font_name>] [-help] [-padding <pixels>]
	       [-rv] [-update <seconds>] [-display displayname]
	       [-geometry geom]
	       [-width clockWidth] [-height clockHeight] [-local localName]
	       [-boston] [-newyork] [-chicago] [-denver] [-la]
	       [-hawaii] [-tokyo] [-sydney] [-london]
	       [-paris] [-frankfurt] [-rio]

OK, OK, Hawaii isn't a city, but you get the point. The timezones used
are taken from tztab in the SYSV case, and from /usr/lib/zoneinfo
otherwise, and as such may or may not be correct (the TZ variable
definitions or the city->timezone mappings).

The -local <localName> option causes a clock labeled with <localName>
using the value of TZ at startup as its timezone.  In addition, a GMT
clock always appears.
