Index: sys/kern/kern_tc.c
===================================================================
--- sys/kern/kern_tc.c
+++ sys/kern/kern_tc.c
@@ -1245,7 +1245,7 @@
  * when we booted.
  */
 void
-tc_setclock(struct timespec *ts)
+tc_setclock(struct timespec *ts, bool abs)
 {
 	struct timespec tbef, taft;
 	struct bintime bt, bt2;
@@ -1254,8 +1254,10 @@
 	nanotime(&tbef);
 	mtx_lock_spin(&tc_setclock_mtx);
 	cpu_tick_calibrate(1);
-	binuptime(&bt2);
-	bintime_sub(&bt, &bt2);
+	if (abs) {
+		binuptime(&bt2);
+		bintime_sub(&bt, &bt2);
+	}
 
 	/* XXX fiddle all the little crinkly bits around the fiords... */
 	tc_windup(&bt);
Index: sys/kern/kern_time.c
===================================================================
--- sys/kern/kern_time.c
+++ sys/kern/kern_time.c
@@ -86,6 +86,7 @@
  */
 
 static int	settime(struct thread *, struct timeval *);
+static int	steptime(struct thread *, struct timeval *, bool abs);
 static void	timevalfix(struct timeval *);
 static int	user_clock_nanosleep(struct thread *td, clockid_t clock_id,
 		    int flags, const struct timespec *ua_rqtp,
@@ -120,7 +121,6 @@
 {
 	struct timeval delta, tv1, tv2;
 	static struct timeval maxtime, laststep;
-	struct timespec ts;
 
 	microtime(&tv1);
 	delta = *tv;
@@ -161,9 +161,17 @@
 		}
 	}
 
+	return (steptime(td, tv, true));
+}
+
+static int
+steptime(struct thread *td, struct timeval *tv, bool abs)
+{
+	struct timespec ts;
+
 	ts.tv_sec = tv->tv_sec;
 	ts.tv_nsec = tv->tv_usec * 1000;
-	tc_setclock(&ts);
+	tc_setclock(&ts, abs);
 	resettodr();
 	return (0);
 }
@@ -405,18 +413,29 @@
 
 	if ((error = priv_check(td, PRIV_CLOCK_SETTIME)) != 0)
 		return (error);
-	if (clock_id != CLOCK_REALTIME)
-		return (EINVAL);
-	if (ats->tv_nsec < 0 || ats->tv_nsec >= 1000000000 ||
-	    ats->tv_sec < 0)
-		return (EINVAL);
-	if (!allow_insane_settime &&
-	    (ats->tv_sec > 8000ULL * 365 * 24 * 60 * 60 ||
-	    ats->tv_sec < utc_offset()))
-		return (EINVAL);
-	/* XXX Don't convert nsec->usec and back */
-	TIMESPEC_TO_TIMEVAL(&atv, ats);
-	error = settime(td, &atv);
+	error = EINVAL;
+	switch (clock_id) {
+	case CLOCK_ADJUST_REALTIME:
+		if (ats->tv_nsec < 0 || ats->tv_nsec >= 1000000000 ||
+		    (!allow_insane_settime && (ats->tv_sec > 3600 ||
+		    ats->tv_sec < -3600)))
+			break;
+		TIMESPEC_TO_TIMEVAL(&atv, ats);
+		error = steptime(td, &atv, false);
+		break;
+	case CLOCK_REALTIME:
+		if (ats->tv_nsec < 0 || ats->tv_nsec >= 1000000000 ||
+		    ats->tv_sec < 0)
+			break;
+		if (!allow_insane_settime &&
+		    (ats->tv_sec > 8000ULL * 365 * 24 * 60 * 60 ||
+		     ats->tv_sec < utc_offset()))
+			error = EINVAL;
+		/* XXX Don't convert nsec->usec and back */
+		TIMESPEC_TO_TIMEVAL(&atv, ats);
+		error = settime(td, &atv);
+		break;
+	}
 	return (error);
 }
 
Index: sys/kern/subr_rtc.c
===================================================================
--- sys/kern/subr_rtc.c
+++ sys/kern/subr_rtc.c
@@ -361,7 +361,7 @@
 	}
 
 	if (ts.tv_sec >= 0) {
-		tc_setclock(&ts);
+		tc_setclock(&ts, true);
 #ifdef FFCLOCK
 		ffclock_reset_clock(&ts);
 #endif
Index: sys/sys/time.h
===================================================================
--- sys/sys/time.h
+++ sys/sys/time.h
@@ -480,6 +480,7 @@
 #define	CLOCK_SECOND	13		/* FreeBSD-specific. */
 #define	CLOCK_THREAD_CPUTIME_ID	14
 #define	CLOCK_PROCESS_CPUTIME_ID	15
+#define	CLOCK_ADJUST_REALTIME	16	/* FreeBSD-specific. */
 #endif
 
 #ifndef TIMER_ABSTIME
Index: sys/sys/timetc.h
===================================================================
--- sys/sys/timetc.h
+++ sys/sys/timetc.h
@@ -88,7 +88,7 @@
 
 u_int64_t tc_getfrequency(void);
 void	tc_init(struct timecounter *tc);
-void	tc_setclock(struct timespec *ts);
+void	tc_setclock(struct timespec *ts, bool abs);
 void	tc_ticktock(int cnt);
 void	cpu_tick_calibration(void);