Merge pull request RIOT-OS#20448 from benpicco/cpu/sam0_common-rtc-0

cpu/sam0_common: RTC: avoid negative month after POR

cpu/sam0_common/periph/rtc_rtt.c

@@ -559,12 +559,18 @@ int rtc_get_alarm(struct tm *time)
         return -1;
     }
 
-    time->tm_mon = alarm.bit.MONTH - 1;
+    time->tm_mon = alarm.bit.MONTH;
     time->tm_mday = alarm.bit.DAY;
     time->tm_hour = alarm.bit.HOUR;
     time->tm_min = alarm.bit.MINUTE;
     time->tm_sec = alarm.bit.SECOND;
 
+    /* struct tm has January as 0, RTC has January as 1 */
+    /* But after power-on reset, all RTC values are 0 - avoid negative month */
+    if (time->tm_mon) {
+        --time->tm_mon;
+    }
+
     return 0;
 }
 
@@ -583,11 +589,18 @@ int rtc_get_time(struct tm *time)
         return -1;
     }
 
-    time->tm_mon = clock.bit.MONTH - 1;
+    time->tm_mon = clock.bit.MONTH;
     time->tm_mday = clock.bit.DAY;
     time->tm_hour = clock.bit.HOUR;
     time->tm_min = clock.bit.MINUTE;
     time->tm_sec = clock.bit.SECOND;
+
+    /* struct tm has January as 0, RTC has January as 1 */
+    /* But after power-on reset, all RTC values are 0 - avoid negative month */
+    if (time->tm_mon) {
+        --time->tm_mon;
+    }
+
     return 0;
 }