diff --git a/os/hal/src/hal_rtc.c b/os/hal/src/hal_rtc.c
index 840e2ec79..c9b280dd5 100644
--- a/os/hal/src/hal_rtc.c
+++ b/os/hal/src/hal_rtc.c
@@ -46,7 +46,11 @@
  * Lookup table with months' length
  */
 static const uint8_t month_len[12] = {
-  31, 30, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+  31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
+};
+
+static const uint16_t accu_month_len[12] = {
+  0, 31, 59,  90, 120, 151, 181, 212, 243, 273, 304, 334
 };
 
 /*===========================================================================*/
@@ -220,7 +224,8 @@ void rtcSetCallback(RTCDriver *rtcp, rtccb_t callback) {
 void rtcConvertDateTimeToStructTm(const RTCDateTime *timespec,
                                   struct tm *timp,
                                   uint32_t *tv_msec) {
-  int sec;
+  int sec, year;
+  bool is_leap_year;
 
   timp->tm_year  = (int)timespec->year + (int)(RTC_BASE_YEAR - 1900U);
   timp->tm_mon   = (int)timespec->month - 1;
@@ -237,6 +242,15 @@ void rtcConvertDateTimeToStructTm(const RTCDateTime *timespec,
   if (NULL != tv_msec) {
     *tv_msec = (uint32_t)timespec->millisecond % 1000U;
   }
+
+  /* Day of the year calculation.*/
+  year = timp->tm_year + 1900;
+  timp->tm_yday = timp->tm_mday - 1;
+  timp->tm_yday += accu_month_len[timp->tm_mon];
+  is_leap_year = ((year % 4 == 0) && (year % 100 != 0)) || (year % 400 == 0);
+  if (is_leap_year && (timp->tm_mon > 1)) {
+    timp->tm_yday++;
+  }
 }
 
 /**
diff --git a/readme.txt b/readme.txt
index 279f650bd..33d08b6dd 100644
--- a/readme.txt
+++ b/readme.txt
@@ -128,6 +128,8 @@
        MEMS Accelerometers.
 - NEW: Safer messages mechanism for sandboxes (to be backported to 20.3.1).
 - NEW: Added latency measurement test application.
+- FIX: Fixed FAT time problem in RTC driver (bug #1142)
+       (backported to 20.3.3)(backported to 19.1.5).
 - FIX: Fixed Heap allocation of aligned FIFO objects in chFactory (bug #1141)
        (backported to 20.3.3)(backported to 19.1.5).
 - FIX: Fixed chsnprintf() sign mode/filler mode conflict (bug #1140)