diff --git a/lib/libsys/_umtx_op.2 b/lib/libsys/_umtx_op.2
--- a/lib/libsys/_umtx_op.2
+++ b/lib/libsys/_umtx_op.2
@@ -210,6 +210,8 @@
 .It
 .Dv CLOCK_SECOND
 .It
+.Dv CLOCK_TAI
+.It
 .Dv CLOCK_UPTIME
 .It
 .Dv CLOCK_UPTIME_FAST
diff --git a/lib/libsys/clock_gettime.2 b/lib/libsys/clock_gettime.2
--- a/lib/libsys/clock_gettime.2
+++ b/lib/libsys/clock_gettime.2
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 28, 2024
+.Dd August 10, 2024
 .Dt CLOCK_GETTIME 2
 .Os
 .Sh NAME
@@ -99,11 +99,20 @@
 Returns the execution time of the calling process.
 .It Dv CLOCK_THREAD_CPUTIME_ID
 Returns the execution time of the calling thread.
+.It Dv CLOCK_TAI
+Increments in SI seconds like a wall clock.
+It uses a 1970 epoch and implements the TAI timescale.
+Similar to CLOCK_REALTIME, except does not include leap seconds and will
+increase monotonically during a leap second.
+Will return EINVAL if the current offset between TAI and UTC is not known,
+which may be the case early in boot before NTP or other time daemon has
+synchronized.
 .El
 .Pp
 The clock IDs
 .Dv CLOCK_BOOTTIME ,
 .Dv CLOCK_REALTIME ,
+.Dv CLOCK_TAI ,
 .Dv CLOCK_MONOTONIC ,
 and
 .Dv CLOCK_UPTIME
@@ -202,7 +211,8 @@
 .Dv CLOCK_MONOTONIC_PRECISE ,
 .Dv CLOCK_REALTIME_FAST ,
 .Dv CLOCK_REALTIME_PRECISE ,
-.Dv CLOCK_SECOND
+.Dv CLOCK_SECOND ,
+.Dv CLOCK_TAI ,
 .Dv CLOCK_UPTIME ,
 .Dv CLOCK_UPTIME_FAST ,
 and
diff --git a/lib/libsys/nanosleep.2 b/lib/libsys/nanosleep.2
--- a/lib/libsys/nanosleep.2
+++ b/lib/libsys/nanosleep.2
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 3, 2022
+.Dd August 10, 2024
 .Dt NANOSLEEP 2
 .Os
 .Sh NAME
@@ -124,6 +124,8 @@
 .It
 CLOCK_SECOND
 .It
+CLOCK_TAI
+.It
 CLOCK_UPTIME
 .It
 CLOCK_UPTIME_FAST
diff --git a/lib/libthr/thread/thr_condattr.c b/lib/libthr/thread/thr_condattr.c
--- a/lib/libthr/thread/thr_condattr.c
+++ b/lib/libthr/thread/thr_condattr.c
@@ -94,6 +94,7 @@
 	if (attr == NULL || *attr == NULL)
 		return (EINVAL);
 	if (clock_id != CLOCK_REALTIME &&
+	    clock_id != CLOCK_TAI &&
 	    clock_id != CLOCK_VIRTUAL &&
 	    clock_id != CLOCK_PROF &&
 	    clock_id != CLOCK_MONOTONIC) {
diff --git a/share/man/man3/pthread_condattr.3 b/share/man/man3/pthread_condattr.3
--- a/share/man/man3/pthread_condattr.3
+++ b/share/man/man3/pthread_condattr.3
@@ -85,7 +85,8 @@
 .Xr pthread_cond_timedwait 3
 and may be set to
 .Dv CLOCK_REALTIME
-(default)
+(default),
+.Dv CLOCK_TAI ,
 or
 .Dv CLOCK_MONOTONIC .
 .Pp
diff --git a/sys/kern/kern_ntptime.c b/sys/kern/kern_ntptime.c
--- a/sys/kern/kern_ntptime.c
+++ b/sys/kern/kern_ntptime.c
@@ -154,6 +154,7 @@
 static int time_state = TIME_OK;	/* clock state */
 int time_status = STA_UNSYNC;	/* clock status bits */
 static long time_tai;			/* TAI offset (s) */
+bool time_tai_set = false;		/* Whether time_tai has been set */
 static long time_monitor;		/* last time offset scaled (ns) */
 static long time_constant;		/* poll interval (shift) (s) */
 static long time_precision = 1;		/* clock precision (ns) */
@@ -388,8 +389,10 @@
 			time_constant = ntv->constant;
 	}
 	if (modes & MOD_TAI) {
-		if (ntv->constant > 0) /* XXX zero & negative numbers ? */
+		if (ntv->constant > 0) { /* XXX zero & negative numbers ? */
 			time_tai = ntv->constant;
+			time_tai_set = true;
+		}
 	}
 #ifdef PPS_SYNC
 	if (modes & MOD_PPSMAX) {
@@ -504,7 +507,7 @@
  * simulation.
  */
 void
-ntp_update_second(int64_t *adjustment, time_t *newsec)
+ntp_update_second(int64_t *adjustment, time_t *newsec, long *tai_off)
 {
 	int tickrate;
 	l_fp ftemp;		/* 32/64-bit temporary */
@@ -624,6 +627,7 @@
 		L_ADD(time_adj, ftemp);
 	}
 	*adjustment = time_adj;
+	*tai_off = time_tai;
 		
 #ifdef PPS_SYNC
 	if (pps_valid > 0)
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -72,6 +72,7 @@
 	uint64_t		th_scale;
 	u_int			th_large_delta;
 	u_int	 		th_offset_count;
+	long			th_tai_offset;
 	struct bintime		th_offset;
 	struct bintime		th_bintime;
 	struct timeval		th_microtime;
@@ -1038,6 +1039,7 @@
 /* Internal NTP status and error estimates. */
 extern int time_status;
 extern long time_esterror;
+extern bool time_tai_set;
 
 /*
  * Take a snapshot of sysclock data which can be used to compare system clocks
@@ -1066,6 +1068,7 @@
 		th = timehands;
 		gen = atomic_load_acq_int(&th->th_generation);
 		fbi->th_scale = th->th_scale;
+		fbi->th_tai_offset = th->th_tai_offset;
 		fbi->tick_time = th->th_offset;
 #ifdef FFCLOCK
 		ffth = fftimehands;
@@ -1139,6 +1142,11 @@
 			getboottimebin(&boottimebin);
 			bintime_add(bt, &boottimebin);
 		}
+		if (!(flags & FBCLOCK_LEAPSEC)) {
+			if (!time_tai_set)
+				return (EINVAL);
+			bt->sec += cs->fb_info.th_tai_offset;
+		}
 		break;
 #ifdef FFCLOCK
 	case SYSCLOCK_FFWD:
@@ -1433,7 +1441,8 @@
 
 		do {
 			t = bt.sec;
-			ntp_update_second(&th->th_adjustment, &bt.sec);
+			ntp_update_second(&th->th_adjustment, &bt.sec,
+			    &th->th_tai_offset);
 			if (bt.sec != t)
 				th->th_boottime.sec += bt.sec - t;
 			--i;
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
@@ -49,6 +49,7 @@
 #include <sys/proc.h>
 #include <sys/posix4.h>
 #include <sys/time.h>
+#include <sys/timeffc.h>
 #include <sys/timers.h>
 #include <sys/timetc.h>
 #include <sys/vnode.h>
@@ -318,7 +319,10 @@
 kern_clock_gettime(struct thread *td, clockid_t clock_id, struct timespec *ats)
 {
 	struct timeval sys, user;
+	struct sysclock_snap clk;
+	struct bintime bt;
 	struct proc *p;
+	int err;
 
 	p = td->td_proc;
 	switch (clock_id) {
@@ -329,6 +333,14 @@
 	case CLOCK_REALTIME_FAST:
 		getnanotime(ats);
 		break;
+	case CLOCK_TAI:
+		sysclock_getsnapshot(&clk, 0);
+		err = sysclock_snap2bintime(&clk, &bt, clk.sysclock_active,
+		    (clk.sysclock_active == SYSCLOCK_FFWD) ? FFCLOCK_LERP : 0);
+		if (err)
+			return (err);
+		bintime2timespec(&bt, ats);
+		break;
 	case CLOCK_VIRTUAL:
 		PROC_LOCK(p);
 		PROC_STATLOCK(p);
@@ -451,6 +463,7 @@
 	case CLOCK_REALTIME:
 	case CLOCK_REALTIME_FAST:
 	case CLOCK_REALTIME_PRECISE:
+	case CLOCK_TAI:
 	case CLOCK_MONOTONIC:
 	case CLOCK_MONOTONIC_FAST:
 	case CLOCK_MONOTONIC_PRECISE:
@@ -515,6 +528,7 @@
 	case CLOCK_REALTIME_PRECISE:
 	case CLOCK_REALTIME_FAST:
 	case CLOCK_SECOND:
+	case CLOCK_TAI:
 		is_abs_real = (flags & TIMER_ABSTIME) != 0;
 		break;
 	case CLOCK_MONOTONIC:
diff --git a/sys/kern/kern_umtx.c b/sys/kern/kern_umtx.c
--- a/sys/kern/kern_umtx.c
+++ b/sys/kern/kern_umtx.c
@@ -686,6 +686,7 @@
 		timo->is_abs_real = clockid == CLOCK_REALTIME ||
 		    clockid == CLOCK_REALTIME_FAST ||
 		    clockid == CLOCK_REALTIME_PRECISE ||
+		    clockid == CLOCK_TAI ||
 		    clockid == CLOCK_SECOND;
 	}
 }
@@ -780,6 +781,7 @@
 	case CLOCK_PROF:
 	case CLOCK_THREAD_CPUTIME_ID:
 	case CLOCK_PROCESS_CPUTIME_ID:
+	case CLOCK_TAI: /* Boot time is not necessarily stable in TAI */
 	default:
 		kern_clock_gettime(curthread, timo->clockid, &timo->cur);
 		if (timespeccmp(&timo->end, &timo->cur, <=))
@@ -2944,8 +2946,9 @@
 			umtx_key_release(&uq->uq_key);
 			return (EFAULT);
 		}
-		if (clockid < CLOCK_REALTIME ||
-		    clockid >= CLOCK_THREAD_CPUTIME_ID) {
+		if ((clockid < CLOCK_REALTIME ||
+		    clockid >= CLOCK_THREAD_CPUTIME_ID) &&
+		    clockid != CLOCK_TAI) {
 			/* hmm, only HW clock id will work. */
 			umtx_key_release(&uq->uq_key);
 			return (EINVAL);
diff --git a/sys/sys/_clock_id.h b/sys/sys/_clock_id.h
--- a/sys/sys/_clock_id.h
+++ b/sys/sys/_clock_id.h
@@ -74,6 +74,10 @@
 #define	CLOCK_PROCESS_CPUTIME_ID 15
 #endif /* __POSIX_VISIBLE >= 199309 */
 
+#ifdef __BSD_VISIBLE
+#define	CLOCK_TAI		16
+#endif
+
 /*
  * Linux compatible names.
  */
diff --git a/sys/sys/timeffc.h b/sys/sys/timeffc.h
--- a/sys/sys/timeffc.h
+++ b/sys/sys/timeffc.h
@@ -89,7 +89,7 @@
  *			of the kernel tick timer (1/hz [s]).
  * FFCLOCK_LERP:	Linear interpolation of ffclock time to guarantee
  *			monotonic time.
- * FFCLOCK_LEAPSEC:	Include leap seconds.
+ * {FB|FF}CLOCK_LEAPSEC: Include leap seconds.
  * {FB|FF}CLOCK_UPTIME:	Time stamp should be relative to system boot, not epoch.
  */
 #define	FFCLOCK_FAST		0x00000001
@@ -100,6 +100,7 @@
 
 #define	FBCLOCK_FAST		0x00010000 /* Currently unused. */
 #define	FBCLOCK_UPTIME		0x00020000
+#define	FBCLOCK_LEAPSEC		0x00040000
 #define	FBCLOCK_MASK		0xffff0000
 
 /*
@@ -111,6 +112,7 @@
 	struct bintime		error;
 	struct bintime		tick_time;
 	uint64_t		th_scale;
+	long			th_tai_offset;
 	int			status;
 };
 
diff --git a/sys/sys/timex.h b/sys/sys/timex.h
--- a/sys/sys/timex.h
+++ b/sys/sys/timex.h
@@ -154,7 +154,7 @@
 #ifdef __FreeBSD__
 
 #ifdef _KERNEL
-void	ntp_update_second(int64_t *adjustment, time_t *newsec);
+void	ntp_update_second(int64_t *adjustment, time_t *newsec, long *tai_off);
 #else /* !_KERNEL */
 #include <sys/cdefs.h>