diff --git a/src/backend/utils/adt/datetime.c b/src/backend/utils/adt/datetime.c
index 793d8a9adcc..ef841a1a454 100644
--- a/src/backend/utils/adt/datetime.c
+++ b/src/backend/utils/adt/datetime.c
@@ -702,9 +702,18 @@ ParseFraction(char *cp, double *frac)
 	}
 	else
 	{
+		/*
+		 * On the other hand, let's reject anything that's not digits after
+		 * the ".".  strtod is happy with input like ".123e9", but that'd
+		 * break callers' expectation that the result is in 0..1.  (It's quite
+		 * difficult to get here with such input, but not impossible.)
+		 */
+		if (strspn(cp + 1, "0123456789") != strlen(cp + 1))
+			return DTERR_BAD_FORMAT;
+
 		errno = 0;
 		*frac = strtod(cp, &cp);
-		/* check for parse failure */
+		/* check for parse failure (probably redundant given prior check) */
 		if (*cp != '\0' || errno != 0)
 			return DTERR_BAD_FORMAT;
 	}
@@ -2964,31 +2973,23 @@ DecodeNumberField(int len, char *str, int fmask,
 	 */
 	if ((cp = strchr(str, '.')) != NULL)
 	{
-		/*
-		 * Can we use ParseFractionalSecond here?  Not clear whether trailing
-		 * junk should be rejected ...
-		 */
-		if (cp[1] == '\0')
-		{
-			/* avoid assuming that strtod will accept "." */
-			*fsec = 0;
-		}
-		else
-		{
-			double		frac;
+		int			dterr;
 
-			errno = 0;
-			frac = strtod(cp, NULL);
-			if (errno != 0)
-				return DTERR_BAD_FORMAT;
-			*fsec = rint(frac * 1000000);
-		}
+		/* Convert the fraction into *fsec */
+		dterr = ParseFractionalSecond(cp, fsec);
+		if (dterr)
+			return dterr;
 		/* Now truncate off the fraction for further processing */
 		*cp = '\0';
 		len = strlen(str);
 	}
+
+	/* What's left had better be all digits */
+	if (strspn(str, "0123456789") != len)
+		return DTERR_BAD_FORMAT;
+
 	/* No decimal point and no complete date yet? */
-	else if ((fmask & DTK_DATE_M) != DTK_DATE_M)
+	if (cp == NULL && (fmask & DTK_DATE_M) != DTK_DATE_M)
 	{
 		if (len >= 6)
 		{
diff --git a/src/test/regress/expected/horology.out b/src/test/regress/expected/horology.out
index b90bfcd794f..5ae93d8e8a5 100644
--- a/src/test/regress/expected/horology.out
+++ b/src/test/regress/expected/horology.out
@@ -467,6 +467,15 @@ SELECT timestamp with time zone 'Y2001M12D27H04MM05S06.789-08';
 ERROR:  invalid input syntax for type timestamp with time zone: "Y2001M12D27H04MM05S06.789-08"
 LINE 1: SELECT timestamp with time zone 'Y2001M12D27H04MM05S06.789-0...
                                         ^
+-- More examples we used to accept and should not
+SELECT timestamp with time zone 'J2452271 T X03456-08';
+ERROR:  invalid input syntax for type timestamp with time zone: "J2452271 T X03456-08"
+LINE 1: SELECT timestamp with time zone 'J2452271 T X03456-08';
+                                        ^
+SELECT timestamp with time zone 'J2452271 T X03456.001e6-08';
+ERROR:  invalid input syntax for type timestamp with time zone: "J2452271 T X03456.001e6-08"
+LINE 1: SELECT timestamp with time zone 'J2452271 T X03456.001e6-08'...
+                                        ^
 -- conflicting fields should throw errors
 SELECT date '1995-08-06 epoch';
 ERROR:  invalid input syntax for type date: "1995-08-06 epoch"
diff --git a/src/test/regress/sql/horology.sql b/src/test/regress/sql/horology.sql
index 1310b432773..8978249a5dc 100644
--- a/src/test/regress/sql/horology.sql
+++ b/src/test/regress/sql/horology.sql
@@ -102,6 +102,10 @@ SELECT date 'J J 1520447';
 SELECT timestamp with time zone 'Y2001M12D27H04M05S06.789+08';
 SELECT timestamp with time zone 'Y2001M12D27H04MM05S06.789-08';
 
+-- More examples we used to accept and should not
+SELECT timestamp with time zone 'J2452271 T X03456-08';
+SELECT timestamp with time zone 'J2452271 T X03456.001e6-08';
+
 -- conflicting fields should throw errors
 SELECT date '1995-08-06 epoch';
 SELECT date '1995-08-06 infinity';