diff --git a/drivers/rtt_rtc/rtt_rtc.c b/drivers/rtt_rtc/rtt_rtc.c
index 3fb2533ed6..e70fd9691a 100644
--- a/drivers/rtt_rtc/rtt_rtc.c
+++ b/drivers/rtt_rtc/rtt_rtc.c
@@ -138,17 +138,18 @@ int rtc_set_time(struct tm *time)
 
 int rtc_get_time_ms(struct tm *time, uint16_t *ms)
 {
-    uint32_t prev = rtc_now;
+    uint32_t tmp, prev = rtc_now;
 
     /* repeat calculation if an alarm triggered in between */
     do {
         uint32_t now = rtt_get_counter();
-        uint32_t tmp = _rtc_now(now);
+        tmp = _rtc_now(now);
 
-        rtc_localtime(tmp, time);
-        *ms = (SUBSECONDS(now) * MS_PER_SEC) / RTT_SECOND;
+        *ms = (SUBSECONDS(now - last_alarm) * MS_PER_SEC) / RTT_SECOND;
     } while (prev != rtc_now);
 
+    rtc_localtime(tmp, time);
+
     return 0;
 }