diff --git a/openbsc/src/gsm_04_11.c b/openbsc/src/gsm_04_11.c
index 05e55d4..16c5c07 100644
--- a/openbsc/src/gsm_04_11.c
+++ b/openbsc/src/gsm_04_11.c
@@ -257,7 +257,7 @@
 	struct tm *tm = localtime(&time);
 
 	*scts++ = bcdify(tm->tm_year % 100);
-	*scts++ = bcdify(tm->tm_mon);
+	*scts++ = bcdify(tm->tm_mon + 1);
 	*scts++ = bcdify(tm->tm_mday);
 	*scts++ = bcdify(tm->tm_hour);
 	*scts++ = bcdify(tm->tm_min);