diff --git a/sys/dev/iicbus/ds3231reg.h b/sys/dev/iicbus/ds3231reg.h
index 86f087ce3095..1564d0960dc4 100644
--- a/sys/dev/iicbus/ds3231reg.h
+++ b/sys/dev/iicbus/ds3231reg.h
@@ -38,8 +38,8 @@
 #define	DS3231_MINS		0x01
 #define	DS3231_MINS_MASK		0x7f
 #define	DS3231_HOUR		0x02
-#define	DS3231_HOUR_MASK_12HR		0x3f
-#define	DS3231_HOUR_MASK_24HR		0x1f
+#define	DS3231_HOUR_MASK_12HR		0x1f
+#define	DS3231_HOUR_MASK_24HR		0x3f
 #define	DS3231_HOUR_IS_PM		0x20
 #define	DS3231_HOUR_USE_AMPM		0x40
 #define	DS3231_WEEKDAY		0x03