doc: add examples to date --help

* src/date.c (usage): Add examples for TZ handling,
and "seconds since epoch" parsing, neither of which
was mentioned in the man page until now.
* THANKS.in: Add Rick.
Suggested by Rick Stanley.

src/date.c

@@ -235,6 +235,18 @@ After any flags comes an optional field width, as a decimal number;\n\
 then an optional modifier, which is either\n\
 E to use the locale's alternate representations if available, or\n\
 O to use the locale's alternate numeric symbols if available.\n\
+"), stdout);
+      fputs (_("\
+\n\
+Examples:\n\
+Convert seconds since the epoch (1970-01-01 UTC) to a date\n\
+  $ date --date='@2147483647'\n\
+\n\
+Show the time on the west coast of the US (use tzselect(1) to find TZ)\n\
+  $ TZ='America/Los_Angeles' date\n\
+\n\
+Show the local time for 9AM next Friday on the west coast of the US\n\
+  $ date --date='TZ=\"America/Los_Angeles\" 09:00 next Fri'\n\
 "), stdout);
       emit_ancillary_info ();
     }