diff --git a/os/hal/ports/STM32/LLD/RTCv2/rtc_lld.c b/os/hal/ports/STM32/LLD/RTCv2/rtc_lld.c
index 940abef7e..51ee7c41c 100644
--- a/os/hal/ports/STM32/LLD/RTCv2/rtc_lld.c
+++ b/os/hal/ports/STM32/LLD/RTCv2/rtc_lld.c
@@ -392,7 +392,7 @@ void rtc_lld_get_time(RTCDriver *rtcp, RTCDateTime *timespec) {
   /* If the RTC is capable of sub-second counting then the value is
      normalized in milliseconds and added to the time.*/
 #if STM32_RTC_HAS_SUBSECONDS
-  subs = (((ssr << 16) / STM32_RTC_PRESS_VALUE) * 1000) >> 16;
+  subs = (((STM32_RTC_PRESS_VALUE - 1U) - ssr) * 1000U) / STM32_RTC_PRESS_VALUE;
 #else
   subs = 0;
 #endif /* STM32_RTC_HAS_SUBSECONDS */
diff --git a/readme.txt b/readme.txt
index 5682306cb..95881022e 100644
--- a/readme.txt
+++ b/readme.txt
@@ -75,6 +75,7 @@
 
 *** 3.0.0p4 ***
 - HAL: New DAC driver implementation for STM32F4xx.
+- HAL: Fixed STM32 RTC SSR Register Counts Down (bug #591).
 - HAL: Fixed STM32 RTC PRER Register not being set in init (bug #590).
 - HAL: Fixed STM32F334 does not have an EXT18 interrupt (bug #588).
 - HAL: Fixed STM32L1xx USB is missing disconnect/connect macros (bug #587).