diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 8f80d9e..70f26b8 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -746,6 +746,11 @@ float8_timestamptz(PG_FUNCTION_ARGS)
 {
 	float8		seconds = PG_GETARG_FLOAT8(0);
 	TimestampTz result;
+#ifdef HAVE_INT64_TIMESTAMP
+	int64       ts_seconds;
+	int64       ts_microseconds;
+#endif
+	int64       epoch_diff_seconds;
 
 	/* Deal with NaN and infinite inputs ... */
 	if (isnan(seconds))
@@ -776,12 +781,14 @@ float8_timestamptz(PG_FUNCTION_ARGS)
 					 errmsg("timestamp out of range: \"%g\"", seconds)));
 
 		/* Convert UNIX epoch to Postgres epoch */
-		seconds -= ((POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * SECS_PER_DAY);
+		epoch_diff_seconds = ((POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * SECS_PER_DAY);
 
 #ifdef HAVE_INT64_TIMESTAMP
-		result = seconds * USECS_PER_SEC;
+		ts_seconds = (int64)seconds;
+		ts_microseconds = (int64)rint((seconds - ts_seconds) * USECS_PER_SEC);
+		result = (ts_seconds - epoch_diff_seconds) * USECS_PER_SEC + ts_microseconds;
 #else
-		result = seconds;
+		result = seconds - epoch_diff_seconds;
 #endif
 
 		/* Recheck in case roundoff produces something just out of range */