diff --git a/src/include/utils/date.h b/src/include/utils/date.h
index 7316ac0ff17..2aca785b65d 100644
--- a/src/include/utils/date.h
+++ b/src/include/utils/date.h
@@ -30,6 +30,14 @@ typedef struct
 	int32		zone;			/* numeric time zone, in seconds */
 } TimeTzADT;
 
+/*
+ * sizeof(TimeTzADT) will be 16 on most platforms due to alignment padding.
+ * However, timetz's typlen is 12 according to pg_type.  In most places
+ * we can get away with using sizeof(TimeTzADT), but where it's important
+ * to match the declared typlen, use TIMETZ_TYPLEN.
+ */
+#define TIMETZ_TYPLEN		12
+
 /*
  * Infinity and minus infinity must be the max and min values of DateADT.
  */