diff --git a/src/STM32RTC.cpp b/src/STM32RTC.cpp
index 1298a9a..3799109 100644
--- a/src/STM32RTC.cpp
+++ b/src/STM32RTC.cpp
@@ -242,13 +242,13 @@ void STM32RTC::enableAlarm(Alarm_Match match, Alarm name)
 #ifdef RTC_ALARM_B
       if (name == ALARM_B) {
         RTC_StartAlarm(::ALARM_B, 0, 0, 0, 0,
-                       (UINT32_MAX - _alarmBSubSeconds), (_alarmBPeriod == AM) ? HOUR_AM : HOUR_PM,
+                       _alarmBSubSeconds, (_alarmBPeriod == AM) ? HOUR_AM : HOUR_PM,
                        static_cast<uint8_t>(31UL));
       } else
 #endif
       {
         RTC_StartAlarm(::ALARM_A, 0, 0, 0, 0,
-                       (UINT32_MAX - _alarmSubSeconds), (_alarmPeriod == AM) ? HOUR_AM : HOUR_PM,
+                       _alarmSubSeconds, (_alarmPeriod == AM) ? HOUR_AM : HOUR_PM,
                        static_cast<uint8_t>(31UL));
       }
       break;
@@ -647,7 +647,7 @@ uint8_t STM32RTC::getAlarmYear(void)
 
 /**
   * @brief  set RTC subseconds.
-  * @param  subseconds: 0-999
+  * @param  subseconds: 0-999 milliseconds
   * @retval none
   */
 void STM32RTC::setSubSeconds(uint32_t subSeconds)
@@ -859,8 +859,7 @@ void STM32RTC::setDate(uint8_t weekDay, uint8_t day, uint8_t month, uint8_t year
   */
 void STM32RTC::setAlarmSubSeconds(uint32_t subSeconds, Alarm name)
 {
-
-  if (getBinaryMode() == MODE_MIX) {
+  if (subSeconds < 1000) {
 #ifdef RTC_ALARM_B
     if (name == ALARM_B) {
       _alarmBSubSeconds = subSeconds;
@@ -871,19 +870,6 @@ void STM32RTC::setAlarmSubSeconds(uint32_t subSeconds, Alarm name)
     {
       _alarmSubSeconds = subSeconds;
     }
-  } else {
-    if (subSeconds < 1000) {
-#ifdef RTC_ALARM_B
-      if (name == ALARM_B) {
-        _alarmBSubSeconds = subSeconds;
-      }
-#else
-      UNUSED(name);
-#endif
-      {
-        _alarmSubSeconds = subSeconds;
-      }
-    }
   }
 }
 
diff --git a/src/rtc.c b/src/rtc.c
index 0f9189c..aa97567 100644
--- a/src/rtc.c
+++ b/src/rtc.c
@@ -76,6 +76,8 @@ static int16_t predivSync = -1;
 static uint32_t prediv = RTC_AUTO_1_SECOND;
 #endif /* !STM32F1xx */
 
+static uint32_t fqce_apre;
+
 static hourFormat_t initFormat = HOUR_FORMAT_12;
 static binaryMode_t initMode = MODE_BINARY_NONE;
 
@@ -326,6 +328,8 @@ static void RTC_computePrediv(int8_t *asynch, int16_t *synch)
     Error_Handler();
   }
   *synch = (int16_t)predivS;
+
+  fqce_apre = clkVal / (*asynch + 1);
 }
 #endif /* !STM32F1xx */
 
@@ -660,8 +664,13 @@ void RTC_GetTime(uint8_t *hours, uint8_t *minutes, uint8_t *seconds, uint32_t *s
 #if defined(RTC_SSR_SS)
     if (subSeconds != NULL) {
       if (initMode == MODE_BINARY_MIX) {
-        *subSeconds = UINT32_MAX - RTC_TimeStruct.SubSeconds;
+        /* The subsecond is the free-running downcounter, to be converted in milliseconds */
+        *subSeconds = (((UINT32_MAX - RTC_TimeStruct.SubSeconds + 1) & UINT32_MAX)
+                       * 1000) / fqce_apre; /* give one more to compensate the 3.9ms uncertainty */
+        *subSeconds = *subSeconds % 1000; /* nb of milliseconds */
+        /* predivAsync is 0x7F with LSE clock source */
       } else {
+        /* the subsecond register value is converted in millisec */
         *subSeconds = ((predivSync - RTC_TimeStruct.SubSeconds) * 1000) / (predivSync + 1);
       }
     }
@@ -731,7 +740,7 @@ void RTC_GetDate(uint8_t *year, uint8_t *month, uint8_t *day, uint8_t *wday)
   * @param hours: 0-12 or 0-23 depends on the hours mode.
   * @param minutes: 0-59
   * @param seconds: 0-59
-  * @param subSeconds: 0-999
+  * @param subSeconds: 0-999 milliseconds
   * @param period: HOUR_AM or HOUR_PM if in 12 hours mode else ignored.
   * @param mask: configure alarm behavior using alarmMask_t combination.
   *              See AN4579 Table 5 for possible values.
@@ -766,7 +775,12 @@ void RTC_StartAlarm(alarm_t name, uint8_t day, uint8_t hours, uint8_t minutes, u
       {
         RTC_AlarmStructure.AlarmSubSecondMask = predivSync_bits << RTC_ALRMASSR_MASKSS_Pos;
       }
-      RTC_AlarmStructure.AlarmTime.SubSeconds = predivSync - (subSeconds * (predivSync + 1)) / 1000;
+      if (initMode == MODE_BINARY_MIX) {
+        /* the subsecond is the millisecond to be converted in a subsecond downcounter value */
+        RTC_AlarmStructure.AlarmTime.SubSeconds = UINT32_MAX - ((subSeconds * fqce_apre) / 1000 + 1);
+      } else {
+        RTC_AlarmStructure.AlarmTime.SubSeconds = predivSync - (subSeconds * (predivSync + 1)) / 1000;
+      }
     } else {
       RTC_AlarmStructure.AlarmSubSecondMask = RTC_ALARMSUBSECONDMASK_ALL;
     }
@@ -835,8 +849,8 @@ void RTC_StartAlarm(alarm_t name, uint8_t day, uint8_t hours, uint8_t minutes, u
         /* Expecting RTC_ALARMSUBSECONDBINMASK_NONE for the subsecond mask on ALARM A */
         RTC_AlarmStructure.AlarmSubSecondMask = mask << RTC_ALRMASSR_MASKSS_Pos;
       }
-      /* Special case for ALARM B configuration when using subsecond reg. in RTC Mix mode */
-      RTC_AlarmStructure.AlarmTime.SubSeconds = subSeconds;
+      /* The subsecond in ms is converted in ticks unit 1 tick is 1000 / fqce_apre */
+      RTC_AlarmStructure.AlarmTime.SubSeconds = UINT32_MAX - (subSeconds * fqce_apre) / 1000;
     }
 
 #else
@@ -933,7 +947,11 @@ void RTC_GetAlarm(alarm_t name, uint8_t *day, uint8_t *hours, uint8_t *minutes,
 #if defined(RTC_SSR_SS)
     if (subSeconds != NULL) {
       if (initMode == MODE_BINARY_MIX) {
-        *subSeconds = UINT32_MAX - RTC_AlarmStructure.AlarmTime.SubSeconds;
+        /*
+         * The subsecond is the bit SS[14:0] of the ALARM SSR register (not ALARMxINR)
+         * to be converted in milliseconds
+         */
+        *subSeconds = (((0x7fff - RTC_AlarmStructure.AlarmTime.SubSeconds + 1) & 0x7fff) * 1000) / fqce_apre;
       } else {
         *subSeconds = ((predivSync - RTC_AlarmStructure.AlarmTime.SubSeconds) * 1000) / (predivSync + 1);
       }