diff --git a/lib/libc/gen/exterr_cat_filenames.h b/lib/libc/gen/exterr_cat_filenames.h
--- a/lib/libc/gen/exterr_cat_filenames.h
+++ b/lib/libc/gen/exterr_cat_filenames.h
@@ -11,6 +11,7 @@
 	[EXTERR_CAT_FILEDESC] = "kern/kern_descrip.c",
 	[EXTERR_CAT_PROCEXIT] = "kern/kern_exit.c",
 	[EXTERR_CAT_FORK] = "kern/kern_fork.c",
+	[EXTERR_CAT_TIME] = "kern/kern_time.c",
 	[EXTERR_CAT_GENIO] = "kern/sys_generic.c",
 	[EXTERR_CAT_VFSBIO] = "kern/vfs_bio.c",
 	[EXTERR_CAT_INOTIFY] = "kern/vfs_inotify.c",
diff --git a/sys/kern/kern_time.c b/sys/kern/kern_time.c
--- a/sys/kern/kern_time.c
+++ b/sys/kern/kern_time.c
@@ -32,10 +32,12 @@
 #include <sys/cdefs.h>
 #include "opt_ktrace.h"
 
+#define	EXTERR_CATEGORY	EXTERR_CAT_TIME
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/limits.h>
 #include <sys/clock.h>
+#include <sys/exterrvar.h>
 #include <sys/lock.h>
 #include <sys/mutex.h>
 #include <sys/sysproto.h>
@@ -216,7 +218,7 @@
 		*clk_id = MAKE_THREAD_CPUCLOCK(tid);
 		return (0);
 	default:
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Invalid CPUCLOCK_WHICH %jd", which));
 	}
 }
 
@@ -300,14 +302,14 @@
 		tid = clock_id & CPUCLOCK_ID_MASK;
 		td2 = tdfind(tid, p->p_pid);
 		if (td2 == NULL)
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "Invalid thread id %jd", tid));
 		kern_thread_cputime(td2, ats);
 		PROC_UNLOCK(td2->td_proc);
 	} else {
 		pid = clock_id & CPUCLOCK_ID_MASK;
 		error = pget(pid, PGET_CANSEE, &p2);
 		if (error != 0)
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "Invalid process id %jd", pid));
 		kern_process_cputime(p2, ats);
 		PROC_UNLOCK(p2);
 	}
@@ -381,7 +383,7 @@
 		break;
 	default:
 		if ((int)clock_id >= 0)
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "Unknown clock id %jd", clock_id));
 		return (get_cputime(td, clock_id, ats));
 	}
 	return (0);
@@ -419,13 +421,13 @@
 	if ((error = priv_check(td, PRIV_CLOCK_SETTIME)) != 0)
 		return (error);
 	if (clock_id != CLOCK_REALTIME)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "clock_id %jd not CLOCK_REALTIME", clock_id));
 	if (!timespecvalid_interval(ats))
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Invalid timespec"));
 	if (!allow_insane_settime &&
 	    (ats->tv_sec > 8000ULL * 365 * 24 * 60 * 60 ||
 	    ats->tv_sec < utc_offset()))
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Insane clock jump"));
 	/* XXX Don't convert nsec->usec and back */
 	TIMESPEC_TO_TIMEVAL(&atv, ats);
 	error = settime(td, &atv);
@@ -493,7 +495,7 @@
 	default:
 		if ((int)clock_id < 0)
 			goto cputime;
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "clock_id %jd invalid", clock_id));
 	}
 	return (0);
 }
@@ -523,9 +525,9 @@
 	bool is_abs_real, precise;
 
 	if (rqt->tv_nsec < 0 || rqt->tv_nsec >= NS_PER_SEC)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "rqt bad nanosecond %jd", rqt->tv_nsec));
 	if ((flags & ~TIMER_ABSTIME) != 0)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Bad timer flags set %#jx", flags));
 	switch (clock_id) {
 	case CLOCK_REALTIME:
 	case CLOCK_TAI:
@@ -562,7 +564,7 @@
 		return (ENOTSUP);
 	case CLOCK_THREAD_CPUTIME_ID:
 	default:
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "clock_id %jd invalid", clock_id));
 	}
 	do {
 		ts = *rqt;
@@ -743,7 +745,8 @@
 	if (tv) {
 		if (tv->tv_usec < 0 || tv->tv_usec >= 1000000 ||
 		    tv->tv_sec < 0)
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "Bad timeval %jd %jd",
+			    tv->tv_sec, tv->tv_usec));
 		error = settime(td, tv);
 	}
 	return (error);
@@ -795,7 +798,7 @@
 	struct timeval ctv;
 
 	if (which > ITIMER_PROF)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Invalid itimer type %ju", which));
 
 	if (which == ITIMER_REAL) {
 		/*
@@ -863,19 +866,22 @@
 		return (kern_getitimer(td, which, oitv));
 
 	if (which > ITIMER_PROF)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Invalid itimer type %ju", which));
 #ifdef KTRACE
 	if (KTRPOINT(td, KTR_STRUCT))
 		ktritimerval(aitv);
 #endif
 	if (itimerfix(&aitv->it_value) ||
 	    aitv->it_value.tv_sec > INT32_MAX / 2)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Interval too large %jd",
+		    aitv->it_value.tv_sec));
 	if (!timevalisset(&aitv->it_value))
 		timevalclear(&aitv->it_interval);
 	else if (itimerfix(&aitv->it_interval) ||
 	    aitv->it_interval.tv_sec > INT32_MAX / 2)
-		return (EINVAL);
+		return (EXTERROR(EINVAL,
+		    "Interval too large for existing timer %jd",
+		    aitv->it_interval.tv_sec));
 
 	if (which == ITIMER_REAL) {
 		PROC_LOCK(p);
@@ -1282,20 +1288,22 @@
 	int error;
 
 	if (clock_id < 0 || clock_id >= MAX_CLOCKS)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Invalid clock_id %jd", clock_id));
 
 	if (posix_clocks[clock_id].timer_create == NULL)
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Null posix clock %jd",clock_id));
 
 	if (evp != NULL) {
 		if (evp->sigev_notify != SIGEV_NONE &&
 		    evp->sigev_notify != SIGEV_SIGNAL &&
 		    evp->sigev_notify != SIGEV_THREAD_ID)
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "sigev_notify invalid %jd",
+			    evp->sigev_notify));
 		if ((evp->sigev_notify == SIGEV_SIGNAL ||
 		     evp->sigev_notify == SIGEV_THREAD_ID) &&
 			!_SIG_VALID(evp->sigev_signo))
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "signal number invalid %jd",
+			    evp->sigev_notify));
 	}
 
 	if (p->p_itimers == NULL)
@@ -1420,7 +1428,7 @@
 	it = itimer_find(p, timerid);
 	if (it == NULL) {
 		PROC_UNLOCK(p);
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "No itimer found id %jd", timerid));
 	}
 	PROC_UNLOCK(p);
 
@@ -1478,7 +1486,7 @@
 	PROC_LOCK(p);
 	if (timer_id < 3 || (it = itimer_find(p, timer_id)) == NULL) {
 		PROC_UNLOCK(p);
-		error = EINVAL;
+		error = EXTERROR(EINVAL, "No itimer found %jd", timer_id);
 	} else {
 		PROC_UNLOCK(p);
 		itimer_enter(it);
@@ -1519,7 +1527,7 @@
 	PROC_LOCK(p);
 	if (timer_id < 3 || (it = itimer_find(p, timer_id)) == NULL) {
 		PROC_UNLOCK(p);
-		error = EINVAL;
+		error = EXTERROR(EINVAL, "No itimer found %d", timer_id);
 	} else {
 		PROC_UNLOCK(p);
 		itimer_enter(it);
@@ -1553,7 +1561,7 @@
 	if (timer_id < 3 ||
 	    (it = itimer_find(p, timer_id)) == NULL) {
 		PROC_UNLOCK(p);
-		error = EINVAL;
+		error = EXTERROR(EINVAL, "No itimer found %jd", timer_id);
 	} else {
 		td->td_retval[0] = it->it_overrun_last;
 		ITIMER_UNLOCK(it);
@@ -1625,11 +1633,11 @@
 
 	val = *value;
 	if (itimespecfix(&val.it_value))
-		return (EINVAL);
+		return (EXTERROR(EINVAL, "Cannot fix itimer"));
 
 	if (timespecisset(&val.it_value)) {
 		if (itimespecfix(&val.it_interval))
-			return (EINVAL);
+			return (EXTERROR(EINVAL, "Cannot fix interval"));
 	} else {
 		timespecclear(&val.it_interval);
 	}
diff --git a/sys/sys/exterr_cat.h b/sys/sys/exterr_cat.h
--- a/sys/sys/exterr_cat.h
+++ b/sys/sys/exterr_cat.h
@@ -40,6 +40,7 @@
 #define	EXTERR_CAT_FORK		15
 #define	EXTERR_CAT_PROCEXIT	16
 #define	EXTERR_CAT_VMM		17
+#define	EXTERR_CAT_TIME		18
 
 #endif