diff --git a/application.c b/application.c
index 20222e3..ebc8dc0 100644
--- a/application.c
+++ b/application.c
@@ -46,6 +46,9 @@ static volatile bool wakeup_flag = false; /**< flag set when wakeup timer trigge
 static volatile bool second_flag = false; /**< flag set when a second passed */
 /** @} */
 
+/** number of seconds since boot */
+static uint32_t boot_time = 0;
+
 size_t putc(char c)
 {
 	size_t length = 0; // number of characters printed
@@ -81,14 +84,36 @@ static void command_version(void* argument)
 	printf("device serial: %08x%08x%08x\n", DESIG_UNIQUE_ID2, DESIG_UNIQUE_ID1, DESIG_UNIQUE_ID0); // show complete serial (different than the one used for USB)
 }
 
+/** convert RTC date/time to number of seconds
+ *  @return number of seconds since 2000-01-01 00:00:00
+ *  @warning for simplicity I consider every month to have 31 days
+ */
+static uint32_t rtc_to_seconds(void)
+{
+	rtc_wait_for_synchro(); // wait until date/time is synchronised
+	const uint8_t year = ((RTC_DR >> RTC_DR_YT_SHIFT) & RTC_DR_YT_MASK) * 10 + ((RTC_DR >> RTC_DR_YU_SHIFT) & RTC_DR_YU_MASK); // get year
+	uint8_t month = ((RTC_DR >> RTC_DR_MT_SHIFT) & RTC_DR_MT_MASK) * 10 + ((RTC_DR >> RTC_DR_MU_SHIFT) & RTC_DR_MU_MASK); // get month
+	if (month > 0) { // month has been initialized, but starts with 1
+		month--; // fix for calculation
+	}
+	uint8_t day = ((RTC_DR >> RTC_DR_DT_SHIFT) & RTC_DR_DT_MASK) * 10 + ((RTC_DR >> RTC_DR_DU_SHIFT) & RTC_DR_DU_MASK); // get day
+	if (day > 0) { // day has been initialized, but starts with 1
+		day--; // fix for calculation
+	}
+	const uint8_t hour = ((RTC_TR >> RTC_TR_HT_SHIFT) & RTC_TR_HT_MASK) * 10 + ((RTC_TR >> RTC_TR_HU_SHIFT) & RTC_TR_HU_MASK); // get hours
+	const uint8_t minute = ((RTC_TR >> RTC_TR_MNT_SHIFT) & RTC_TR_MNT_MASK) * 10 + ((RTC_TR >> RTC_TR_MNU_SHIFT) & RTC_TR_MNU_MASK); // get minutes
+	const uint8_t second = ((RTC_TR >> RTC_TR_ST_SHIFT) & RTC_TR_ST_MASK) * 10 + ((RTC_TR >> RTC_TR_SU_SHIFT) & RTC_TR_SU_MASK); // get seconds
+	const uint32_t seconds = ((((((((year * 12) + month) * 31) + day) * 24) + hour) * 24) + minute) * 60 + second; // convert to number of seconds
+	return seconds;
+}
+
 /** show uptime
  *  @param[in] argument no argument required
  */
 static void command_uptime(void* argument)
 {
 	(void)argument; // we won't use the argument
-	// TODO calculate time difference
-	const uint32_t uptime = 0; // get time from internal RTC
+	const uint32_t uptime = rtc_to_seconds() - boot_time; // get time from internal RTC
 	printf("uptime: %u.%02u:%02u:%02u\n", uptime / (24 * 60 * 60), (uptime / (60 * 60)) % 24, (uptime / 60) % 60, uptime % 60);
 }
 
@@ -410,6 +435,7 @@ void main(void)
 		rtc_lock(); // protect RTC register against writing
 		pwr_enable_backup_domain_write_protect(); // re-enable protection now that we configured the RTC clock
 	}
+	boot_time = rtc_to_seconds(); // remember the start time
 	puts("OK\n");
 
 	// setup wakeup timer for periodic checks