diff --git a/libc/kernel/uapi/linux/rtc.h b/libc/kernel/uapi/linux/rtc.h
index 3312f69..8e70eba 100644
--- a/libc/kernel/uapi/linux/rtc.h
+++ b/libc/kernel/uapi/linux/rtc.h
@@ -69,6 +69,7 @@
 #define RTC_VL_BACKUP_LOW _BITUL(1)
 #define RTC_VL_BACKUP_EMPTY _BITUL(2)
 #define RTC_VL_ACCURACY_LOW _BITUL(3)
+#define RTC_VL_BACKUP_SWITCH _BITUL(4)
 #define RTC_VL_READ _IOR('p', 0x13, unsigned int)
 #define RTC_VL_CLR _IO('p', 0x14)
 #define RTC_IRQF 0x80