diff --git a/src/smblib/smbdefs.h b/src/smblib/smbdefs.h
index 899575e9049fb37ed4cec3c34bfc5f1ec10f8f69..6c289c0e5dc3c021fe8d7176b9926adbb830ae4c 100644
--- a/src/smblib/smbdefs.h
+++ b/src/smblib/smbdefs.h
@@ -368,7 +368,7 @@ typedef struct {		/* Time with time-zone */
 } when_t;
 
 // We encode Month/Day/Hour/Min/Sec into 26 bits:
-#define SMB_DATE_MK_MASK(width, shift)	(((1 << (width + 1)) - 1) << shift)
+#define SMB_DATE_MK_MASK(width, shift)	(((1 << (width)) - 1) << shift)
 #define SMB_DATE_MON_SHIFT		22
 #define SMB_DATE_MON_MASK		SMB_DATE_MK_MASK(4, SMB_DATE_MON_SHIFT)
 #define SMB_DATE_DAY_SHIFT		17