Add -u option to date (POSIX and Plan 9, and useful)

date.c

@@ -11,21 +11,27 @@ main(int argc, char *argv[])
 	char buf[BUFSIZ], c;
 	char *fmt = "%c";
 	struct tm *now = NULL;
+	struct tm *(*tztime)(const time_t *) = localtime;
+	const char *tz = "local";
 	time_t t;
 
 	t = time(NULL);
-	while((c = getopt(argc, argv, "d:")) != -1)
+	while((c = getopt(argc, argv, "d:u")) != -1)
 		switch(c) {
 		case 'd':
 			t = estrtol(optarg, 0);
 			break;
+		case 'u':
+			tztime = gmtime;
+			tz = "gm";
+			break;
 		default:
 			exit(EXIT_FAILURE);
 		}
 	if(optind < argc && argv[optind][0] == '+')
 		fmt = &argv[optind][1];
-	if(!(now = localtime(&t)))
-		eprintf("localtime failed\n");
+	if(!(now = tztime(&t)))
+		eprintf("%stime failed\n", tz);
 
 	strftime(buf, sizeof buf, fmt, now);
 	puts(buf);