diff --git a/timer.c b/timer.c
index 3daf79c..c6806a2 100644
--- a/timer.c
+++ b/timer.c
@@ -69,8 +69,11 @@ static void timer_update_nearest_alarm(Timer *tmr) {
     AlarmAssignment *nearest = NULL; // nearest alarm
     for (uint32_t i = 0; i < tmr->maxSched; i++) {
         AlarmAssignment * iter = tmr->alarms + i;
+        if (iter->id == 0) { // do not consider empty slots
+            continue;
+        }
         int64_t t_i_us = time_to_us(&(iter->time));
-        if ((nearest == NULL) && (iter->id != 0)) { // if it's the first one
+        if (nearest == NULL) { // if it's the first one
             nearest = iter;
             min_delta_t_us = t_i_us - t_c_us;
         } else { // if it's not the first one
@@ -103,6 +106,7 @@ uint32_t timer_sched(Timer *tmr, const TimePoint *t, TimerAlarmCb cb, AlarmUserD
 
     // increase allocation count
     tmr->nSched++;
+    MSG("%d %s\n", tmr->nSched, __FUNCTION__);
 
     // replace nearest if needed
     if (tmr->nSched > 1) {
@@ -132,6 +136,7 @@ void timer_unsched(Timer *tmr, uint32_t id) {
         if (alarm != NULL) {
             memset(alarm, 0, sizeof(AlarmAssignment));
             tmr->nSched--;
+            MSG("%d %s\n", tmr->nSched, __FUNCTION__);
             timer_update_nearest_alarm(tmr);
         }
     }
@@ -176,12 +181,11 @@ void timer_tick(Timer *tmr, int64_t us) {
 
             // decrease scheduled alarm count
             tmr->nSched--;
+            MSG("%d %s\n", tmr->nSched, __FUNCTION__);
 
             // update nearest alarm
             timer_update_nearest_alarm(tmr);
 
-            ETHLIB_OS_MTX_UNLOCK(&(tmr->tabMtx));
-
             // invoke callback
             if (alarm.cb != NULL) {
                 alarm.cb(tmr, alarm.params);
@@ -190,6 +194,8 @@ void timer_tick(Timer *tmr, int64_t us) {
             if (tmr->nextAlarm != NULL) {
                 t_alarm = time_to_us(&(tmr->nextAlarm->time));
             }
+
+            ETHLIB_OS_MTX_UNLOCK(&(tmr->tabMtx));
         }
     }
 }
diff --git a/timer.h b/timer.h
index 4878dfc..5c697f6 100644
--- a/timer.h
+++ b/timer.h
@@ -10,8 +10,8 @@
  * A single point in time
  */
 typedef struct {
-    uint32_t s; ///< Seconds
-    uint32_t us; ///< Microseconds
+    int64_t s; ///< Seconds
+    int64_t us; ///< Microseconds
 } TimePoint;
 
 /**