diff --git a/src/rtc.c b/src/rtc.c
--- a/src/rtc.c
+++ b/src/rtc.c
@@ -34,8 +34,8 @@ void rtc_init(void)
 
 	hrtc.Instance = RTC;
 	hrtc.Init.HourFormat = RTC_HOURFORMAT_24;
-	hrtc.Init.AsynchPrediv = 124;
-	hrtc.Init.SynchPrediv = 322; // if this has enough bits should be 1.0018Hz based on 40kHz LSI
+	hrtc.Init.AsynchPrediv = 127; // Note that both these dividers actually divide by their value + 1
+	hrtc.Init.SynchPrediv = 311; // About 1Hz with 40kHz LSI
 	hrtc.Init.OutPut = RTC_OUTPUT_DISABLE;
  	hrtc.Init.OutPutPolarity = RTC_OUTPUT_POLARITY_HIGH;
 	hrtc.Init.OutPutType = RTC_OUTPUT_TYPE_OPENDRAIN;
@@ -102,6 +102,58 @@ RTC_TimeTypeDef* rtc_time(void)
 	return &time_last;
 }
 
+uint16_t __rtc_millis(uint32_t sub_seconds)
+{
+	// Subsecond counter counts down from SynchPrediv value to zero
+	return (hrtc.Init.SynchPrediv - sub_seconds) * 999 / hrtc.Init.SynchPrediv; // 0 - 999 mS
+}
+
+#define JULIAN_DATE_BASE 2440588 // Unix epoch time in Julian calendar (UnixTime = 00:00:00 01.01.1970 => JDN = 2440588)
+
+uint64_t rtc_timestamp(void)
+{
+	RTC_TimeTypeDef time = {0};
+	RTC_DateTypeDef date = {0};
+
+	HAL_RTC_GetTime(&hrtc, &time, RTC_FORMAT_BCD);
+	HAL_RTC_GetDate(&hrtc, &date, RTC_FORMAT_BCD);
+
+	// Thanks to LonelyWolf: https://github.com/LonelyWolf/stm32/blob/master/stm32l-dosfs/RTC.c
+	// Convert Date/Time structures to epoch time
+	uint8_t  a;
+	uint16_t y;
+	uint8_t  m;
+	uint64_t JDN;
+
+	// These hardcore math's are taken from http://en.wikipedia.org/wiki/Julian_day
+
+	// Calculate some coefficients
+	a = (14 - date.Month) / 12;
+	y = (date.Year + 2000) + 4800 - a; // years since 1 March, 4801 BC
+	m = date.Month + (12 * a) - 3; // since 1 March, 4801 BC
+
+	// Gregorian calendar date compute
+	JDN  = date.Date;
+	JDN += (153 * m + 2) / 5;
+	JDN += 365 * y;
+	JDN += y / 4;
+	JDN += -y / 100;
+	JDN += y / 400;
+	JDN  = JDN - 32045;
+	JDN  = JDN - JULIAN_DATE_BASE;    // Calculate from base date
+	JDN *= 86400;                     // Days to seconds
+	JDN += time.Hours * 3600;    // ... and today seconds
+	JDN += time.Minutes * 60;
+	JDN += time.Seconds;
+
+	JDN *= 1000; // Prepare to add ms
+
+	JDN += __rtc_millis(time.SubSeconds);
+
+	return JDN;
+
+}
+
 
 void rtc_cal(void)
 {