diff --git a/TODO-RELEASE b/TODO-RELEASE
index 9f5240f50..41bd5d83e 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -11,6 +11,7 @@ core      ADD       osmo_sock_multiaddr_{add,del}_local_addr()
 core      ADD       osmo_sock_multiaddr_get_ip_and_port(), osmo_multiaddr_ip_and_port_snprintf(), osmo_sock_multiaddr_get_name_buf()
 core      ADD       osmo_sock_sctp_get_peer_addr_info()
 core      ADD       gsmtap_inst_fd2() core, DEPRECATE gsmtap_inst_fd()
+core		behavior change		osmo_tdef_fsm_inst_state_chg(): allow millisecond precision
 isdn		ABI change		add states and flags for external T200 handling
 gsm		ABI change		add T200 timer states to lapdm_datalink
 gsm		ABI change		add UI queue to struct lapdm_datalink
diff --git a/include/osmocom/core/tdef.h b/include/osmocom/core/tdef.h
index d9d267511..402d01029 100644
--- a/include/osmocom/core/tdef.h
+++ b/include/osmocom/core/tdef.h
@@ -121,11 +121,13 @@ struct osmo_tdef_state_timeout {
 const struct osmo_tdef_state_timeout *osmo_tdef_get_state_timeout(uint32_t state,
 								  const struct osmo_tdef_state_timeout *timeouts_array);
 
-/*! Call osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer(), depending on the timeouts_array, tdefs and
- * default_timeout.
+/*! Call osmo_fsm_inst_state_chg[_ms]() or osmo_fsm_inst_state_chg_keep_timer[_ms](),
+ * depending on the timeouts_array, tdefs and default_timeout.
  *
- * A T timer configured in sub-second precision is rounded up to the next full second. A timer in unit =
- * OSMO_TDEF_CUSTOM is applied as if the unit is in seconds (i.e. this macro does not make sense for custom units!).
+ * A timer defined with sub-millisecond precision (e.g OSMO_TDEF_US) is rounded up to the next full millisecond.
+ * A timer value defined in units higher than millisecond (e.g. OSMO_TDEF_S, OSMO_TDEF_M) is converted to milliseconds.
+ * A timer in unit = OSMO_TDEF_CUSTOM is applied as if the unit is in seconds (i.e. this macro does not make sense
+ * for custom units!).
  *
  * See osmo_tdef_get_state_timeout() and osmo_tdef_get().
  *
@@ -153,9 +155,10 @@ const struct osmo_tdef_state_timeout *osmo_tdef_get_state_timeout(uint32_t state
  * \param[in] state  State number to transition to.
  * \param[in] timeouts_array  Array of struct osmo_tdef_state_timeout[32] to look up state in.
  * \param[in] tdefs  Array of struct osmo_tdef (last entry zero initialized) to look up T in.
- * \param[in] default_timeout  If a T is set in timeouts_array, but no timeout value is configured for T, then use this
- *                             default timeout value as fallback, or pass -1 to abort the program.
- * \return Return value from osmo_fsm_inst_state_chg() or osmo_fsm_inst_state_chg_keep_timer().
+ * \param[in] default_timeout  If a T is set in timeouts_array, but no timeout value is configured for T,
+ *                             then use this default timeout value (in seconds) as fallback,
+ *                             or pass a negative number to abort the program.
+ * \return Return value from osmo_fsm_inst_state_chg[_ms]() or osmo_fsm_inst_state_chg_keep_timer[_ms]().
  */
 #define osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout) \
 	_osmo_tdef_fsm_inst_state_chg(fi, state, timeouts_array, tdefs, default_timeout, \
diff --git a/src/core/tdef.c b/src/core/tdef.c
index abbe581aa..f0c0f2e80 100644
--- a/src/core/tdef.c
+++ b/src/core/tdef.c
@@ -337,26 +337,37 @@ int _osmo_tdef_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t state,
 				  const char *file, int line)
 {
 	const struct osmo_tdef_state_timeout *t = osmo_tdef_get_state_timeout(state, timeouts_array);
-	unsigned long val = 0;
+	unsigned long val_ms = 0;
 
 	/* No timeout defined for this state? */
 	if (!t)
 		return _osmo_fsm_inst_state_chg(fi, state, 0, 0, file, line);
 
-	if (t->T)
-		val = osmo_tdef_get(tdefs, t->T, OSMO_TDEF_S, default_timeout);
+	if (t->T) {
+		const struct osmo_tdef *tdef = osmo_tdef_get_entry((struct osmo_tdef *)tdefs, t->T);
+		if (tdef == NULL) {
+			/* emulate the old behavior: treat default_timeout as OSMO_TDEF_S */
+			OSMO_ASSERT(default_timeout >= 0);
+			val_ms = default_timeout * 1000;
+		} else {
+			val_ms = osmo_tdef_round(tdef->val, tdef->unit, OSMO_TDEF_MS);
+			/* emulate the old behavior: treat OSMO_TDEF_CUSTOM as OSMO_TDEF_S */
+			if (tdef->unit == OSMO_TDEF_CUSTOM)
+				val_ms *= 1000;
+		}
+	}
 
 	if (t->keep_timer) {
 		if (t->T)
-			return _osmo_fsm_inst_state_chg_keep_or_start_timer(fi, state, val, t->T, file, line);
+			return _osmo_fsm_inst_state_chg_keep_or_start_timer_ms(fi, state, val_ms, t->T, file, line);
 		else
 			return _osmo_fsm_inst_state_chg_keep_timer(fi, state, file, line);
 	}
 
-	/* val is always initialized here, because if t->keep_timer is false, t->T must be != 0.
+	/* val_ms is always initialized here, because if t->keep_timer is false, t->T must be != 0.
 	 * Otherwise osmo_tdef_get_state_timeout() would have returned NULL. */
 	OSMO_ASSERT(t->T);
-	return _osmo_fsm_inst_state_chg(fi, state, val, t->T, file, line);
+	return _osmo_fsm_inst_state_chg_ms(fi, state, val_ms, t->T, file, line);
 }
 
 const struct value_string osmo_tdef_unit_names[] = {
diff --git a/tests/tdef/tdef_test.err b/tests/tdef/tdef_test.err
index 1a39b8961..8e5860d7c 100644
--- a/tests/tdef/tdef_test.err
+++ b/tests/tdef/tdef_test.err
@@ -1,10 +1,10 @@
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){A}: Allocated
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){A}: State change to A (T1, 100s)
-DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){A}: State change to B (T2, 1s)
+DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){A}: State change to B (T2, 100ms)
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){B}: State change to C (T3, 3000s)
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){C}: State change to D (T4, 100s)
-DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){D}: State change to E (X5, 1s)
-DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){E}: State change to F (X6, 1s)
+DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){D}: State change to E (X5, 100ms)
+DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){E}: State change to F (X6, 1ms)
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){F}: State change to G (T7, 50s)
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){G}: State change to H (T8, 300s)
 DLGLOBAL DEBUG tdef_test(test_tdef_state_timeout){H}: State change to I (T9, 300s)
diff --git a/tests/tdef/tdef_test.ok b/tests/tdef/tdef_test.ok
index d934292fa..827b9ab35 100644
--- a/tests/tdef/tdef_test.ok
+++ b/tests/tdef/tdef_test.ok
@@ -165,11 +165,11 @@ osmo_tdef_get(7, s)	= 50
 test_tdef_state_timeout()
 state=A T=0, no timeout
  --> A (configured as T1 100 s) rc=0;	state=A T=1, 100.000000 s remaining
- --> B (configured as T2 100 ms) rc=0;	state=B T=2, 1.000000 s remaining
+ --> B (configured as T2 100 ms) rc=0;	state=B T=2, 0.100000 s remaining
  --> C (configured as T3 50 m) rc=0;	state=C T=3, 3000.000000 s remaining
  --> D (configured as T4 100 custom-unit) rc=0;	state=D T=4, 100.000000 s remaining
- --> E (configured as T-5 100 ms) rc=0;	state=E T=-5, 1.000000 s remaining
- --> F (configured as T-6 100 us) rc=0;	state=F T=-6, 1.000000 s remaining
+ --> E (configured as T-5 100 ms) rc=0;	state=E T=-5, 0.100000 s remaining
+ --> F (configured as T-6 100 us) rc=0;	state=F T=-6, 0.001000 s remaining
  --> G (configured as T7 50 s) rc=0;	state=G T=7, 50.000000 s remaining
  --> H (configured as T8 300 s) rc=0;	state=H T=8, 300.000000 s remaining
  --> I (configured as T9 5 m) rc=0;	state=I T=9, 300.000000 s remaining