Index: sbin/dmesg/dmesg.8
===================================================================
--- sbin/dmesg/dmesg.8
+++ sbin/dmesg/dmesg.8
@@ -78,7 +78,10 @@
 The default value is shown next to each variable.
 .Bl -tag -width indent
 .It  kern.msgbuf_show_timestamp : No 0
-If set to 1, then a timestamp will be added to most lines in the message buffer.
+If set to 0, no timetamps are added.
+If set to 1, then a 1-second granularity timestamp will be added to most lines
+in the message buffer.
+If set to 2, then a microsecond granularity timestamp will be added.
 This may also be set as a boot
 .Xr loader 8
 tunable.
Index: sys/kern/subr_msgbuf.c
===================================================================
--- sys/kern/subr_msgbuf.c
+++ sys/kern/subr_msgbuf.c
@@ -234,8 +234,16 @@
 
 		if (msgbuf_show_timestamp && needtime == 1 &&
 		    (mbp->msg_flags & MSGBUF_NEEDNL) == 0) {
-			snprintf(buf, sizeof(buf), "[%jd] ",
-			    (intmax_t)time_uptime);
+			if (msgbuf_show_timestamp == 1) {
+				snprintf(buf, sizeof(buf), "[%jd] ",
+				    (intmax_t)time_uptime);
+			} else {
+				struct timeval tv;
+
+				microuptime(&tv);
+				snprintf(buf, sizeof(buf), "[%jd.%06d] ",
+				    (intmax_t)tv.tv_sec, (int)tv.tv_usec);
+			}
 			for (j = 0; buf[j] != '\0'; j++)
 				msgbuf_do_addchar(mbp, buf[j]);
 			needtime = 0;