As some people have shown an interest in this (see httpd: add timestamps
to fastcgi error log), here is an initial version for switching syslogd
to ISO8601 timestamps by default.

Personally, I would welcome it. So far, I have only tested it locally. I
have NO larger syslogd server up and running.

Rafael

diff --git a/usr.sbin/syslogd/parsemsg.c b/usr.sbin/syslogd/parsemsg.c
index a28ac9752c5..a2e0d345bb0 100644
--- a/usr.sbin/syslogd/parsemsg.c
+++ b/usr.sbin/syslogd/parsemsg.c
@@ -27,7 +27,7 @@
 #include "parsemsg.h"
 #include "syslogd.h"
 
-size_t parsemsg_timestamp_bsd(const char *, char *);
+size_t parsemsg_timestamp_bsd(const char *);
 size_t parsemsg_timestamp_v1(const char *, char *);
 size_t parsemsg_prog(const char *, char *);
 
@@ -41,7 +41,8 @@ parsemsg(const char *msgstr, struct msg *msg)
 	if (msg->m_pri &~ (LOG_FACMASK|LOG_PRIMASK))
 		msg->m_pri = -1;
 
-	if ((n = parsemsg_timestamp_bsd(msgstr, msg->m_timestamp)) == 0)
+	msg->m_timestamp[0] = '\0';
+	if ((n = parsemsg_timestamp_bsd(msgstr)) == 0)
 		n = parsemsg_timestamp_v1(msgstr, msg->m_timestamp);
 	msgstr += n;
 
@@ -83,21 +84,18 @@ parsemsg_priority(const char *msg, int *pri)
 }
 
 size_t
-parsemsg_timestamp_bsd(const char *msg, char *timestamp)
+parsemsg_timestamp_bsd(const char *msg)
 {
 	size_t i;
 
-	timestamp[0] = '\0';
 	for (i = 0; i < 16; i++) {
 		if (msg[i] == '\0')
 			return 0;
 	}
 
-	if (msg[3] == ' ' && msg[6] == ' ' && msg[9] == ':' && msg[12] == ':' &&
-	    msg[15] == ' ') {
-		/* BSD syslog TIMESTAMP, RFC 3164 */
-		if (!ZuluTime)
-			strlcpy(timestamp, msg, 16);
+	if (msg[3] == ' ' && msg[6] == ' ' && msg[9] == ':' &&
+	    msg[12] == ':' && msg[15] == ' ') {
+		/* BSD syslog TIMESTAMP, RFC 3164, ignored */
 		return 16;
 	}
 
diff --git a/usr.sbin/syslogd/syslogd.8 b/usr.sbin/syslogd/syslogd.8
index 038be0574f3..fb7fd8b2232 100644
--- a/usr.sbin/syslogd/syslogd.8
+++ b/usr.sbin/syslogd/syslogd.8
@@ -177,9 +177,8 @@ attacks over the network, including attackers remotely filling logs.
 Do not perform remote server certificate and hostname validation
 when sending messages.
 .It Fl Z
-Generate timestamps in ISO format.
-This includes the year and the timezone, and all logging is done
-in UTC.
+This option is ignored and only accepted for backward compatibility.
+Timestamps are always generated in ISO 8601 format in UTC.
 .El
 .Pp
 The options
diff --git a/usr.sbin/syslogd/syslogd.c b/usr.sbin/syslogd/syslogd.c
index af76ec2e181..eefcee99feb 100644
--- a/usr.sbin/syslogd/syslogd.c
+++ b/usr.sbin/syslogd/syslogd.c
@@ -229,7 +229,6 @@ int	MarkSeq = 0;		/* mark sequence number */
 int	Repeat = 0;		/* 0 msg repeated, 1 in files only, 2 never */
 int	SecureMode = 1;		/* when true, speak only unix domain socks */
 int	NoDNS = 0;		/* when true, refrain from doing DNS lookups */
-int	ZuluTime = 0;		/* display date and time in UTC ISO format */
 int	IncludeHostname = 0;	/* include RFC 3164 hostnames when forwarding */
 int	Family = PF_UNSPEC;	/* protocol family, may disable IPv4 or IPv6 */
 
@@ -484,8 +483,7 @@ main(int argc, char *argv[])
 		case 'V':		/* do not verify certificates */
 			NoVerify = 1;
 			break;
-		case 'Z':		/* time stamps in UTC ISO format */
-			ZuluTime = 1;
+		case 'Z':		/* OBSOLETE, UTC ISO is the default */
 			break;
 		default:
 			usage();
@@ -1771,21 +1769,17 @@ struct timeval	now;
 void
 current_time(char *timestamp)
 {
-	(void)gettimeofday(&now, NULL);
-
-	if (ZuluTime) {
-		struct tm *tm;
-		size_t l;
+	struct tm *tm;
+	size_t l;
 
-		tm = gmtime(&now.tv_sec);
-		l = strftime(timestamp, 33, "%FT%T", tm);
-		/*
-		 * Use only millisecond precision as some time has
-		 * passed since syslog(3) was called.
-		 */
-		snprintf(timestamp + l, 33 - l, ".%03ldZ", now.tv_usec / 1000);
-	} else
-		strlcpy(timestamp, ctime(&now.tv_sec) + 4, 16);
+	(void)gettimeofday(&now, NULL);
+	tm = gmtime(&now.tv_sec);
+	l = strftime(timestamp, 33, "%FT%T", tm);
+	/*
+	 * Use only millisecond precision as some time has
+	 * passed since syslog(3) was called.
+	 */
+	snprintf(timestamp + l, 33 - l, ".%03ldZ", now.tv_usec / 1000);
 }
 
 /*
diff --git a/usr.sbin/syslogd/syslogd.h b/usr.sbin/syslogd/syslogd.h
index b7c630da299..b9bf7e41e86 100644
--- a/usr.sbin/syslogd/syslogd.h
+++ b/usr.sbin/syslogd/syslogd.h
@@ -23,8 +23,6 @@
 
 #include <stdarg.h>
 
-extern int	ZuluTime;
-
 /* Privilege separation */
 void  priv_init(int, int, int, int, char **);
 __dead void priv_exec(const char *, int, int, int, char **);